I voted 4Ed.
My primary reason: I'm part of a campaign that has been active since the mid-1980s, and has been updated with each edition change...except 4Ed. Conversion would involve too many fundamental changes to too many PCs- spell changes, class changes and even lopping off classes from some*- that it would wreck the feel.
And of course, there's more: the truncation of the alignment system, removing alignment as a meaningful force in the game, the excision of Vancian magic, etc.
Certainly, 3Ed changed the game a great deal. That is unarguable. But to me, the change wrought by 4Ed transmogrified the game into something that was D&D in name only. It's still an enjoyable, well-designed game to play, but to me it jettisoned too much of what separated it from all those other FRPGs out there. It lost it's unique flavor. And that makes the 4Ed change bigger.**
* not because the class functionality was mimicked more efficiently by 4Ed classes- which would have been fine- but purely because of 4Ed's multiclassing rules.
** and yet not big enough. I've come to the conclusion that if 4Ed had been released as a new FRPG "from the makers of D&D," it could have been bigger than it is. Released from the shackles of legacy concerns, the 4Ed design could have been so much more. IMHO, of course.