Hands down I believe that the best edition is D&D 3.5. This is the edition that I grew up playing. I have played AD&D, 3.0, 4th, and I play tested D&D Next/5.0. So I think that I have enough experience to justify why 3.5 is the best of all the editions I have played.
In 3.5 the game is at its most balanced, the monsters will not simply destroy you like in the earlier editions. The players also will have a challenge unlike in the editions post 3.5. The main problems with Dungeons and Dragons 3.5 were addressed in the release of a “new” game called Pathfinder. Basically this is 3.5 re released just cleaned up and fixed to make it the best it could be. I will never play D&D again after finding Pathfinder because it took my favorite edition and just made it a better overall game. Complicated enough for experienced players and easy enough for new players.