Original Dungeons & Dragons (incl. BECMI) 21
Advanced Dungeons & Dragons (1e, 2e) 19
Dungeons & Dragons 3.X 16
Dungeons & Dragons 4th edition 3
Dungeons & Dragons 5th edition 23
5th has actually brought me back to D&D from strictly home brew, so that gets my +1 no question.
If I was voting my least favorite edition I'd just kill 4e for being too MMO for me. But I want to give its fans a chance to cast spare the dying first.
So I'll hit AD&D; it gave me many great games, but it is such a bloated, kludgey mess of a game system. And it lacks the extra old school charm of B/X and OD&D. Sorry Gygax, you had some good ideas but I think you were a kinda like Lucas or Shyamalan... When given too much free reign you overbuild your art and it just becomes a little incoherent.