For me it is almost always going to be the most recent edition. I like the general evolution of the game (for the most part) and don't really like to go back. I don't see D&D in the same that I do, for instance, see film or music, where there isn't a clear correlation between quality and recency. I love music from all eras (am particularly fond of 70s funk and jazz) and most of my favorite films were from before 2000.
Now if the question was, "What single D&D book would you choose for forever bathroom reading?" The answer would be the 1E DMG. If you asked, "Which D&D book did you get the most pleasure out of when it first came out?" The answer would be the 1E Deities & Demigods, which I used to bring with me to school and read during recess with my friends, way back in the early 80s.
But as far as the poll, the answer is 5E. It isn't perfect, and I hope to see this and that tweaked, but it does the best job at maintaining classic tropes + streamlined rules + newish bells and whistles.