[name_f]Guinevere[/name_f] is my personal favorite, actually, although I’d still spell the nickname [name_f]Gwen[/name_f]. (Actually, I’d use [name_f]Winnie[/name_f], but that’s just me. [name_f]Gwen[/name_f] would be my second choice.)
I like both [name_f]Gwendolyn[/name_f] and [name_f]Gwendoline[/name_f] on paper, but I’m not sure how to pronounce the latter name.
I don’t like [name_f]Gwyneth[/name_f].