I'd say the most annoying character currently in the sopranos is definatley Janice.
Howvever, the most annoying character who's ever appeard in the show is Chris's meth-head friend in episode 2. I was truly glad to see him whacked. "Fuck Tony Soprano, thats a quote", after that he could never redeem himself in my books.

Personally i loved Ralphie, slightly annoying at times (beating that "whore" ), but he was insanley whitty also (the Ginny Sack remark ), and he was possibly the most nihillistic Capo ever.