I'm going to go with 3.5 as my favorite edition. I really enjoy the mechanics, especially the feats and skills. To me, 3.5 gives you the flexibility to really have fun with character (and monster) design.
Nothing against the Holmes rules or 1E, as I had years of fun with those. We probably played 2E the most, including our modified 2E rules that I like to call our precursor to 3E. There was a skill point system and everything! Once again, my lack of a scanner is killing me - I would love to post a scan of our chart.
4E was a complete and total disaster, and I have to take the blame for that. I convinced the guys to give it a try, as I thought it looked promising. We tried it, and it was an epic fail. I just couldn't get over how much it slowed the game down. I thought it was supposed to speed things up!
I will give D&D Next/5E a good, long look, but it would have to really blow us all away to get us to switch from 3.5 at this point in time.