I don't actually have an answer for this one. I don't think there's one outfit that I particularly associate with 'the best times' - when I think of the times when I've had the most fun in the last few years, I'm sure I wore different outfits for every occasion!
I think the most fun 'nights out' were summer balls (when I wore evening dresses, obviously), Halloween (when I wore costumes!), and friends' birthdays. I always try to wear something 'new' (usually just new to me rather than actually knew, being the charity shop addict that I am) for friends' birthday celebrations as well as my own, and if I haven't got anything 'new' to wear then I pick something that those friends haven't seen me in before. I'm not sure why, although it's quite common for most other people at the party or dinner to be wearing something new!