... Meh, dunno about nicest character ever, but one of fav heroes (a very kind one) is the true beast: Aotsuki Ushio (from Ushio to Tora). Not saying he's the nicest chara I've seen, but one of the few I actually liked lol I liked his characterization a lot, and it's not like he was an angel, which made it even better for me. He was nice not by nature but by will. A great man imo.
Oh, yes...Ushio and....Shakugan (Sengoku Youko) for me ^_^
About evil...well, dunno, perhaps Dio Brando (Jojo's Bizarre Adventrues) or Friend (20th Century boys)...or wait, Kai is more fitting (Dorohedoro). There might be some characters more evil than those, but they're pretty legit for me...