Kanto Starters

Upon Reaching the Luminose City, you can grab yourself the 2nd starter Pokemon, that is the Kanto Starter!

We probably haven't seen them since Pokemon Red/Blue/Green and Fire Red/Leaf Green. Now is the time for Bulbasaur, Charmander and Squirtle to shine again in Kalos region, with the new power that is introduced in Gen 6, the Mega Evolution. Still not sure which one to take among the 3?  Then here is your reference that might help you to decide your 2nd starter!

Bulbasaur

Bulbasaur can evolve to Venusaur that is a Grass/Poison type pokemon.You might also want to take note that before even evolving to Venusaur, Bulbasaur is already a Grass/Poison type Pokemon. It is the only starter Pokemon that has dual type in basic stage. Venusaur evolution line has a pretty solid Special Attack and Special Defense but fairly average for all other stat.

Prior to Gen6, Poison is pretty useless attack type as the type that weak against Poison is not many, which is Grass type ONLY, and Steel is totally immune to Poison type moves. However, with the newly added Fairy type, which is weak against ONLY poison and steel type moves, poison type has become slightly useful when using it in the battle.

After it evolves into Venusaur at Level 32, he can learn a pretty strong grass move that is the Petal Dance. Petal Dance will keep attacking the opponent 2-3 turns with 120 base power. However, the user will become confused afterwards. Since you can have the choice to switch out your battling Pokemon after you defeat each of your opponent pokemon in-game, you can just switch it out Venusaur to another Pokemon to cure the confusion status before sendoing it out again.

As for Venusaur moveset, he can learn alot of Special Condition move such as Sleep powder which is very useful for catching pokemon, Stun Spore which put opponent into paralyzed while reducing opponent's speed to cover Venusaur's average speed.  Despite being a part of Poison type Pokemon, it is quite sad that Bulbasaur (even as Venusaur) can't learn any Poison type attack (except for Poison Powder) by himself. However, before you reach the second gym, you can get Venoshock TM and teach it to your Bulbasaur evolution line. As you proceed further, you can get yourself a Sludge Bomb TM which is a Poison powerful attack.Besides that, Venusaur can't really learn a lot of good move from TM to cover his weakness unlike the other 2 starters.

Mega Venusaur

When Venusaur evolve into Mega Venusaur, he will increase his defense significantly and also slightly increase his Special Defense and special attack. There is no change in its speed. However, what make Mega Venusaur useful is its ability. From Overgrow (which increase Grass type move 50% when Venusaur's HP is below 1/3) to Thick fat, which is able to reduce Fire and Ice moves damage receive by Mega Venasaur. Due to the new ability, Venasaur will now be able to take x1 damage from Fire and Ice moves damage instead of x2 (super effective) damage. Meaning to say now Mega Venusaur only weak against Flying type and Psychic type moves! Only 2 out of the 18 types!

It is also worth to mention that by increasing the Defense and Special defense for Mega Venusaur and its new ability, Venusaur can now become a tanker. Venuasur can endure some hits before he get down although this is compensate by its average speed. His High special attacker also can also OTK a lot of opponent in-game except for Pokemon that resists it. If you really want a tanker, you may consider to go with Mega Venusaur.

Charmander

Charmander, a lizard like Pokemon which will then evolve to a Dragon-like Pokemon, Charizard which has Flying type as its secondary type. Due to its Fire/Flying typing, this poor lizard has x4 weakness to Rock, which means Rock type attack can easily OTK him. Worse still, there's quite a number of non-Rock type Pokemon that can learn rock type move, especially Fighting type pokemon. However, look at the bright side, with flying type, Charizard is immue to Ground type attack which is usually a weakness for Fire type is pokemon. This is quite handful because the popular Earthquake (Ground type move) will not touch it.

Charizard is one of the most popular Pokemon probably due to its look. Appearance aside, why not we look at its stat for now? Charizard has a decent Speed and Special attack, but fairly low defense while the other stats are quite average.

Although being part of flying type Pokemon, the only Flying type move that Charizard can learn is Wing attack, which will has low attack base (only 60) and not to mention that Chazird has higher Sp.ATk compared to its Atk. (Note: Wing attack is Physical move). However, Charizard can learn Air Slash through move relearner. Which mean you will need to bring a heart scale to Dendemille town (which is after the 6th gym) to teach your Charizard this move. This Special attack move has a higher attack base and perfectly match Charizard which is a Special attacker. It is also possible to make opponent flinch for one turn. Sweet! 

As usual, as a fire type Pokemon, Charizard can learn Flamethrower, which is very powerful fire attack that can knock out a lot of pokemons as long as they don't resist fire attack. Charizard can also access to Heatwave, which has higher attack power. It can hit all opponent in Double battle but compensate with a lower accuracy, which is 90% compare to Flamethrower (100%).

As for TM, Charizard can learn Solarbeam, which has high attack base grass type move that can be used to counter Water type and Rock type Pokemon. Even without STAB, Charizard is able to knock off most Water and Rock Pokemon in one shot. Roost is also helpful move to recover Charizard's HP up to 1/2. However it will take off Charizard's Flying type in return. So be careful when your opponent can Ground type attack as it will hit Charizard very hard due to its Fire type.

Along with Mewtwo, Charizard is the only two Pokemon that can access to two types of Mega evolution. You can get Charizardite X in Pokemon X and Charizardite in Pokemon Y. So indirectly you can ONLY use Charizard X in Pokemon X and Charizard Y in Pokemon Y. However you can still trade the stone from another version, if you can get one. ;)

Mega Charizard X

After evolve to Mega Charizard X, it will greatly increase its Attack stat and slightly increase on its Sp.Atk and defense stat. Interesting enough, Nintendo finally grants the wish of the fans. The typing of Charizard becomes Fire/Dragon instead of Fire/Flying. Well, the typing is good since now Charizard can get rid of its x4 Rock weakness, even though there is still x2. Mega Charizard X will also say good bye to its Water and Lighting weakness. However, its is now weak to Dragon.

Mega Charizard X has the same amount of base stat for both Attack and Sp.Atk which is pretty waste due to its ability, Tough Claw. Contact move used by Mega Charizard X is increase by 30% (from what i heard, still haven't confirm the amount). This ability obviously make it as a Physical attacker, which means there's a waste of 130 points on its Sp.Atk stat.

Mega Charizard X can use Dragon Claw for STAB, and with its ability Touch Claw, it can hit even harder!

Not only that, Charizard also can access to Flare Blitz which is one of the most powerful fire type attack, with its high power base (130) and ability, it will hit very hard just like a beast! Mega Charizard X can also learn Earthquake (Note: Keep in mind it's Physical attack but it does not make contact, so Tough claw won't work) and Shadow Claw through TM. If you think the Attack for Mega Charizard X is still not enough, maybe you can try Sword Dance to increase its atk stage twice!

Mega Charizard Y

After evolve to Mega Charizard Y, its Sp.Atk will dramatically increase to the point it is on par with normal Mewtwo. Just imagine how beastly is Mega Charizard Y. Sp.Atk aside, the Attack and Sp.Def of Charizard also increase slightly. Sadly, there's no increase in speed for Charizard.

Well, this did not bring Charizard down, with its new ability, Drought, it will create free Sunny Day when you Mega evolve it. Sunny Day will increase 50% for fire attacks and decrease 50% for water type attacks which is very helpful for Charizard. Not only that, Charizard can unleash Solarbeam the turn it uses without any charging! Basically, Mega Charizard Y can almost OTK almost any same-level Pokemon in game provided you choose correct attack.

Squirtle

A turtle-like Pokemon that will eventually evolve into Blastoise, which is a pure Water type pokemon. This is the only starter among the 3 Kanto starter is pure type, meaning to say it does not have a secondary type which has its good and bad. The Good thing is Blastoise is only weak against Grass and Electric types, while the bad thing is it kinda limits Blastoise's moveset and cause it to be lack of flexibility. Blastoise is a defensive Pokemon with high defense and Sp.Def, but in return, he has low attack and speed compared to the other two.

Blastoise can learn very limited move set by itself as almost all of its move set is water-based. Surprisingly though, Blastoise can learn Flash Cannon, which can be used to counter the new Fairy type Pokemon which is only weak to Steel and Poison types, through move relearner. Flash Cannon has a fairy high power base too!  TM wise, like other Water type Pokemon, Blastoise can access to Ice Beam which can counter Grass type Pokemon. You might want to try Focus Blast, which can be used to hit Normal type Pokemon. Focus Blast has a very high power base but has low accuracy. Blastoise can also learn Rain Dance to increase water type moves by 50% while decreasing Fire type moves 50%. Personally I think it is not really that useful. First of all, Blastoise itself is able to resist Fire type moves, so it is quite meaningless to decrease it even further. Also, Blastoise might want to use its turn to attack instead of set up the rain weather due to its low speed. Thunder will also have 100% accuracy in rain which will indirectly help your opponent to knock out Blastoise even easier.

Mega Blastoise

After Mega evolve to Mega Blastoise, Blastoise will dramatically increase in its Special Attack and slightly increase the other stats except for Speed. Mega Blastoise will also gain a new ability, Mega Launcher. Mega Launcher increase Mega Blastoise "Pulse" moves by 50%. The good thing is Blastoise can access to Dark Pulse. 50% makes a huge difference as it indirectly gives Mega Blastoise STAB when using Dark Pulse. As for water type moves, Mega Blastoise can learn Water Pulse by itself or Surf through HM. Both has its own strength and weakness. Power wise, it is same due to Water Pulse power base increased by 50% (from 60 to 90), which is same to Surf. The good things for Water Pulse is that it has a chance to make opponent confuse while surf can hit all opponent in horde or even double battle (Note: Be aware that it will also hit your ally in double battle).