No contest. After having played it for over a year, both as a DM and player, 4E is by far my favorite edition of D&D, and holds together the best during play for my groups.
As a DM, I love 4e for:
* Ease of preparation (and the DDI is a thing of beauty)
* Simplicity of NPC and monster stat blocks, but still with tons of interesting options during a fight
* Skill challenges (yes, I use them, and they work well especially if the DM uses some judgement)
* Focus on actually playing and running the game, rather than prepping for it
* MUCH harder for a player to munchkin powergame. It means my job as a DM is a ton easier not having to worry about appropriate encounters and challenges trashing some PCs while other PCs don't break a sweat
* The system is so robust and scales well through all levels of play
* XP budgeting system for encounter design
* New cosmology- it strongly mirrors the cosmology of my homebrew world, and I find the new cosmology far more evocative than the old one
As a player, I love 4e for:
* Easier organic character development (meaning I don't need to pre-select feats, skills, classes, etc out to level X to be able to qualify for something later), and retraining
* Focus on play mastery rather than system mastery
* Focus on working as a team, rather than lone-wolf tactics
* MUCH harder to munchkin powergame, and fewer one-trick pony PCs
* I can play ANY class and know I won't be a gimp or useless at any point during the character's progression
* The new multiclassing rules
* Powers are just a stupid amount of fun in play
* Caster class power is reigned in, but a smart caster with some prep time for rituals and careful power use can do some really incredible stuff
* Much less reliance on magic items for character progression and power