OD&D, B/X: Never played.
BECMI: My introduction to the game. I liked the relative simplicity of it all, and I also liked the way that the boxes gradually built on one another, allowing the game to become more involved as we gained experience with it. (I should note that I never got to the "MI" part of this version - moved to 2nd Ed.)
1st Ed: This edition seems to have the bulk of the best adventures, and also seems to be the place where many of the iconic concepts that we now take for granted originated. Even the relative rawness of it is something of a plus - it's unlikely that something like "Expedition to the Barrier Peaks" would be tried now if it hadn't been done before. So, lots of creativity here.
2nd Ed: Possibly controversial, but IMO 2nd Ed is better than 1st - largely because the writing is much cleaner and the organisation much better. (If I'd graduated from BECMI to 1st Ed it's likely I would not now be a gamer.) The expanded format of monsters used in the Monstrous Compendium (though not the folders!) was a highlight. But the best thing about the edition was the settings: for me, it was Ravenloft, Spelljammer, and Dark Sun that were the highlights.
3e/3.5e: On reading the 3e PHB it was immediately obvious to me that this was the edition I never knew I always wanted. Obviously, over time the weaknesses in the mechanics of this edition became sorely apparent, but just having a set of consistent mechanics was a massive step forward. Also, while 2nd Ed really shone in its settings, it was 3e that gave us my absolute favourite in Eberron, as well as the single best setting book I've seen in the FRCS.
4e: In most ways my least-favourite edition (hey, something has to be), 4e had a lot of good ideas, many of which have been sadly dropped. The Bloodied condition is something I still use. But my favourite was the explicitly-stated monster roles and the Solo/Elite/regular/Minion split - that really helped with encounter design.
5e: In many ways I find this to be a truer successor to 2nd Ed than the previous two editions, in a good way - it has the cleaned up mechanics, but also lacks the complexity of a 3e or 4e. But right now the single biggest highlights are the "Starter Set" (for "Lost Mine of Phandelver") and the "Essentials Kit" (for everything except the included adventure).