Most of the Pokémon who can learn Belch are downright awful, mediocre at best, but some can make good of it.
Magmortar
Magmortar@ Sitrus Berry
Ability: Vital Spirit
EVs: 252 SpA / 252 Spe / 4 HP
Modest
- Belch
- Fire Blast
- Thunderbolt
- Hidden Power [Ice]
A base 120 power move coming off of Magmortar's base 125 Sp. Attack is nothing to laugh about, especially for unexpecting Fairies and Grass Pokémon. Fire Blast is STAB, Thunderbolt is coverage and HP Ice hits Ground-types. Any Berry can be used, but Sitrus has use outside of Belch.
Victreebel@ Sitrus Berry
Ability: Chlorophyll
EVs: 252 SpA / 252 Spe / 4 HP
Modest/Timid Nature
- Belch
- Giga Drain
- Hidden Power [Fire]/Weather Ball
- Growth/Sleep Powder
This is for use in the Sun. Belch is a wicked STAB. Giga Drain for recovery. Either HP Fire or Weather Ball is usefull in the Sun, but remember that Weather Ball is more gimmicky. Growth is used for boosting Sp. Atttack, while Sleep Powder is to annoy. Any Berry can be used, but Sitrus has use outside of Belch.
Slowking@ Sitrus Berry
Ability: Regenerator
EVs: 252 SpA / 252 HP / 4 SDef
- Belch
- Psyshock
- Slack Off
- Scald
This is meant to be a surprise set. Lure out any dangerous Grass- or Fairy-types, then hammer tham with Belch. Psyshock and Scald are STAB, and Slack Off is recovery. This is a gimmicky set, and it is not recommended that you use it unless you have to. Any Berry could work, but Sitrus has use outside of Belch.
That's pretty much it. Honourable mentions are Weezing, Seviper and Garbador. Belch isn't the best or most reliable STAB or coverage move, as it is gimmicky and disabled by the ever-present Knock Off.