It depends on what role you need for the team. Vileplume is more of an attacker, while Bellossom is more of a wall. One thing's for sure, you should wait until Level 39 to evolve Gloom, because that's when it learns Toxic. Here's a set for each:
(No EVs, items, or nature because this is in-game)
Vileplume
-Giga Drain (level up)
-Toxic (level up)
-Venoshock (TM, available early-game)
-Infestation (TM, available early-game)
Bellossom
-Giga Drain (level up)
-Toxic (level up)
-Moonlight (level up)
-Infestation (TM, available early-game)