Best motivation? I'm going with Elpizo and a few others on this one and say its Xande, Kuja, and Vayne. All three had rather sympathetic motives, Vayne especially could be considered an anti-hero rather than a villain.

Sure some of the other villains have more "fun" motives but "fun" and "good" are two different things. Kefka and Ex-Death are more symbolic as villains rather than having good motives but it explains why these two are definetly the most evil of the batch.

Worst motive goes to Seymour since the game never really nailed down what it was since it seemed like it changed every time you met him.