Personally, I like unless almost only in statement modifier position, where the expression is simple. This is a matter of taste and I'm not arguing that this is better. (Well, I'm almost certain to wrinkle my nose at *complex* expressions that start with unless.) So I'd much rather do #2 than #4. You'll notice that I included the existence test only in #1, which was the long form.

#5 is fine and dandy, and has the additional feature of giving you a count of seen instances. I do this sometimes :) but where what I'm implemented is strictly a set, sometimes I prefer decidedly not to have it, just to make clear that the only information this hash provides is membership. YMMV etc.