Mega Gengar
Pros:
- Shadow Tag is an overpowered ability that makes the opponent unable to switch out, this helps it be a very efficient Destiny Bond user and sweeper.
- Extremely diverse movepool that includes: Dazzling Gleam, Evergy Ball, Psychic, Dark Pulse and more.
- Very high SpA (170) and Speed (130) stats.
Cons:
- Low Defenses and HP.
- Unlinke ordinary Gengar, it isnt immune to Ground, meaning it has more weaknesses.
Mega Alakazam
Pros:
- Its SpA (175) and Speed (150) stats are amongst the highest in the game.
- While its movepool isnt as diverse as MGengar's, it includes powerful moves such as: Dazzling Gleam, Energy Ball and Focus Blast.
Cons:
- It gets a pretty useless ability, Trace.
- It doesnt have dual type, meaning it only gets STAB from 1 type.
- Just like MGengar, it has low HP and Defense
For your team I would recommend Gengar, but it depends on which one synergises better.
