A nature doesn't really matter in a playthrough, however if you really do care, Modest Nature is one of the worst natures for Crobat since its Special movepool is terrible, but it is better Gen 4 onwards because of new coverage options and the physical/special split giving it a Special Sludge Bomb.
Competitive moveset:
Crobat @ Leftovers / Life Orb
Ability: Inner Focus
EVs: 4 Def / 252 SpA / 252 Spe
- Nasty Plot
- Heat Wave
- Sludge Bomb
- Giga Drain / Air Slash
(This set is usually the same for most generations).
I assume you are going to be playing at the Battle Spot/Stadium. I'll be covering singles here.
Gen 6
The Modest nature is fine. However, do note that Crobat is outclassed by Gengar and Mega Gengar, since they are much more immediately powerful, and Mega Gengar can trap as well.
Gen 7
Once again, Modest is fine. However, now there is almost no reason at all to use Crobat, since its role as a Nasty Plot Poison type was taken by Naganadel.
Gen 8
Same as Gen 7.