Siegfried's always been pretty cool; helps that he's a fairly easy (if cheap : p) character to use. Plus it's nice to see that he at least realistically aged for SC V; rockin' that long hair and stubble all along.
Xianghua's also a fun character to use, though not the biggest fan of her myself. I usually just use her style to make Aqua from Kingdom Hearts. : P
Nightmare, his style of complete domination compliments my agressive playstyle very well. I also like his design, my favorite being his Alt. in SCV, but I prefer the regular Soul Edge to the Alt. version.
This is a tough one. Oh yes. Olcadan was my favorite in 3, because he's a man with an OWL HEAD. Nothing is cooler. Nothing. :3
In four, that's gotta be Ashlotte, I loved how she was essentially a faster Astaroth, and remember thinking "Yes, finally, his style is usable!" Plus the whole Clockwork puppet/maiden thing is pretty dang cool.
I've also always loved playing as Cervantes, and it seems that they really upped his cool factor in 5. He's far more pirate-y and far less... undead, I guess. Good stuff there. biggrin
My favorite fighting style was Talim's, but considering she didn't return in V, I have grown on Viola's fighting style.
I don't have an exact favorite character, but I do like Viola, Tira, Setsuka, and Patroklos.