I didn't really like any edition after 2E, but I did like how they finally fixed that 'AC runs backwards' thing...
Yeah we manually fixed that in late 2E manually. It looks from old character sheets like we did that in 1998, but I thought we did it in reaction to 3E being announced, which was 1999 (I believe). Maybe we just heard rumours, and one of the rumours was flipped AC/THAC0?
We did it for everything - ACs, To Hit (can't call it THAC0 any more!), saves, proficiencies. It was a tremendous improvement.
But anyway favourite things from non-favourite editions. 2E I listed as my favourite, so we'll skip that - it makes sense too, because I'd have too many things.
1E - Cavaliers. Unearthed Arcana in general. How they did Bards, even though it's really stupid it's also really cool. So of the adventures are wonderful too (probably not the ones you're thinking of!).
RC D&D - Weapon mastery. Oh my god why did no other D&D after that do that? It solved so many problems and potentially could massively narrowed the caster-muggle gap. Also single-part alignments are imho objectively better than two-part alignments.
3E - The changes to how saves worked. I think this was the best way of having it - Will, Fortitude and Reflex, and 3E came up with that. The sheer willingness to just continually add more and more and more and more material into the pot. Yeah, a lot of it was terrible, terrible misses, or just stupid OP (still mad about PrCs that are "objectively superior" to certain base classes, i.e. they got everything the base class did and more!), but there loads of hits too, and loads of just really cool and weird stuff.
4E - The lore - loads of people have mentioned it, but the Dawn War/Primordials, the Feywild/Shadowfell, and so on. It was really good stuff and way overdue I feel. Making Tieflings core, because you all knew it was coming. A lot of cool races in general. Amazing classes that we still haven't got back and probably won't until the last year or two of 5E, or in 6E. How it made players who used to be bad at understanding how their characters worked into sudden experts, who really got it. How amazingly easy it was to DM.
5E - The cool simplicity of a lot of it. The elegance, I guess. The remarkable combination of good traits from various editions. Subclasses and how well-executed they are. Full-caster Bards (as a habitual Bard player this was like some Charles Atlas-type deal!). Warlocks, even though I don't play them. Not backsliding on the 4E races (much).