Abomasnow-Mega
Pros
- Solid HP
- Excellent 132 Atk and SpA
- Highly defensive with 105 Def and SpD
- Gets Powerful STAB moves like Wood Hammer and Blizzard
Cons
- Very slow (base 30 Speed)
- 7 weaknesses, including a 4x weakness to Fire
Abomasnow is really strong and bulky when Mega Evolved.
Lapras
Pros
- A useful ability in Water Absorb make you Immune to Water, which you will encounter in the Elite Four
- 130 HP is just amazing
- Better Def and SpD
- Gets more coverage moves
Cons
- Bit slow but faster than Abomasnow
- Hard to get in wild (though you can easily get one as a gift)
- Offensive stats are worse than Mega Abomasnow's
Lapras is also good in a playthrough.
Even when Mega-Abomasnow has more pros than Lapras, just go with Lapras. You can Mega Evolve Charizard which, is just awesome. Your moveset should consist of physical attacks for Mega Charizard X and I recommend to replace Flamethrower with Flare Blitz, which deals the highest damage with Tough Claws.
So;
Lapras @ Mystic Water
Ability: Water Absorb
- Surf / Hydro Pump
- Ice Beam
- Thunderbolt
- Psychic
Surf / Hydro Pump for STAB. Surf for is better for accuracy (and it's an HM) but if you already have a Surf user you can go for Hydro Pump to deal more damage.
Ice Beam for STAB and to counter Dragon types.
Thunderbolt and Psychic are for type coverage.