I am running 5e DnD, not 5.5, because I don't think 5.5 improved the game more than it made changes I dislike.
I prefer 1e Paranoia, because it's perfect and the skill system is almost bureaucratic, which is a brilliant thing to do for Paranoia.
I prefer 2e Gamma World; it has just the right amount of gonzo craziness for me, and later editions' mechanics aren't better, just different (IMHO). That said, I do love the version based on 4e DnD, but think it makes for a better one or two shot, rather than a campaign.