Best Marth - Why you ask? First of all, his are fast and well paced and puts good pressure on your opponent. Has also good air attacks, including his meteor smash.
Second would be his counter move, i'm a master with its counter, especially when my opponent is under pressure. Only this fails sometimes with Wi-Fi if there is lag. It ruins my timing. And there is the final smash, one hit, one kill, HISSATSU! whats not to like about that?
Worst - Olimar...to think there would be a character worse then jigglypuff...