I'd like to second amoongus for B-Rank. While I haven't played UU in a bit, Scraftyisthebest is spot on with its qualities. Sadly fire types, certain sub-boosting sets (especially steel types), and Xatu cause the mushroom quite the headache which affects its usefulness somewhat. A possible move, although inferior to doublepowder, for amoongus to use is foul play. This can help against some boosters who use substitute sets, and deal very significant damage to switch-ins. Status is nicer, but late game when clauses and such are in effect, its an interesting option nonetheless.