Flashbang
Pros: Blinds target, deafens and slows.
Cons: Must be looking at grenade or in its direction to have any effect.

Stun
Pros: Deafens, slows and does a little damage to target even if detonated behind.
Cons: Target can still see where he or she is shooting.

Smoke
Pros: Can temporarily provide cover for getaways, relocation and traps (for example, throw a smoke grenade into a room and plant a claymore in the doorway, then throw a grenade inside to flush him out).