Forgive the "stereotyping"
Forgive the "stereotyping" of the different settings, but to make my point, I'm going to go to "typifying" the settings and rulessets as I see them.
First I'll describe the differences between second, third, and fourth:
Second: good, smooth, non mini based play. Relied on description and adjudication heavily...not great balance on rules, but not easily min/maxed to death either. High focus on story, lower focus on rules and options (until that atrocious "player's option" series came out...ugh).
Third: the most simulationist. The real advent of using minis pretty solidly. not really required, like 4e, but still important and helpful. Very complex rules that could do just about anything, including making monster characters, breaking the game, or changing every single class or monster if need be.
Fourth: the most gamist. Very smooth play...smoother than second. More limited versatility overall, (both in terms of options right now as it is new and in terms of a character's options per day...well, at least the spellcasters).
Now on to the settings:
Ravenloft: benefitted from a simulationist perspective, IMO. Best edition for it would be third. The Sword and Sorcery stuff was GREAT! That includes Masque of the red death which was sort of 3.5, but also sort of its own edition in a way...the magic rules were not PHB rules.
Eberron: I'd be 4e is best. Eberron had action points, some bizarre new creatures, and a number of other fantastic things. The lack of utility spells in 4e is not an issue (I know there are rituals, but they are not as diverse as the utility in 3e). Artificers and the "techno-magic" easily allow for needed plot points...need to fly? No problem. Plus the "feel" I think will be better in 4e in the sense of HEROISM (intentionally all capitals). Eberron has, to me, always been about pulp heroism more than gritty heroism. If I wanted to play indiana jones in a WotC setting, this would be it.
Dark Sun: Don't hate me, but I think second edition was best for this. Third allows too much power creep/options, and the grit of the world encourages players to go for whatever they can. Conversely, a lot of the basics of a good dark sun campaign were mere survival...4e seems to skip over the nitty gritty basics of this sort of material (again, the "utility spells" and abilities).
Planescape: I think third would have been best if it had been better supported. The later "affiliations" that were developed (DMG2?) that gave bonuses for achieving a number of things within an organization were perfect for factions. Planescape was all about tons of crazy options...and blending of worlds Even if the rule system wasn't ideal in 3e for planescape, the pure glut of material was. I could planehop from the Arcanis Setting to Golarion to Dragonlance to the hells to a homebrew and so on.
Others:
Alqadim: I'd say 4e would be best...high fantasy swashbuckling 40 thieves/aladin stuff.
Maztica: 2nd edition. not a lot of rules needed, but 4e is too heroic for the setting IMO
Dragonlance: not sure. I suppose I'd choose 4th based only on the dragons finally coming into their own, IMO.
Arcanis and Iron Kingdoms: both 3e. The publishers think so. I think so for my own reasons.
And with all of that said, I'm sure that most of those settings would do just fine in any other edition, including the unknown and nonexistent 5th edition. But my thoughts are on where I believe them to be best.