I like Ezio and Viola the best. Never really got the hang of Viola and I don't like Soulcalibur too much, though... Ezio's awesome though. I like that to deal massive damage you have to time your attacks so that they are counter attacks. He's got some neat combos too.
Though, before I stopped playing, I dropped Viola and started using the... Ninja Chick as my main. Forget her name. She's great though, but it also felt like she was friggin' cheap.