I don't know many good 'A' moves. Pretty much anyone who has fast combos. Link up+A and down+A in the air are pretty good though.

As for 'B' moves, Metaknight's 'B' attack with the tornado is one of the few 'B' moves that's hard to counter and can get you out of close-combat situations. Kirby's down+B in the air is a very powerful move, but doesn't always work against highly-skilled opponents since all they do is shield, grab, and throw. It works well against 'cruel' opponents though.