Dan over at Sword and Board posted his rankings of the D&D versions he's played: D&D editions: How I rate them. I thought it would be fun to do likewise -- or at least how I would rank them today as a couple of these seem to vary over time. I've been playing and running D&D games since 1975. I was 18 years when started to play.
#1: BECMI/RC. This is my favorite published version of D&D. It feels like an "advanced" version of the original D&D I started with. Lots of material to work with, but the rules are loose and seem designed for the GM to modify to fit his campaign. The Rule Cyclopedia has flaws, but it's probably the best set of rules for D&D ever published: everything you need for years of gaming in one book.
#2: OD&D and Supplements. OD&D with its 4 supplements and material from the TSR and GW magazines is a great game. It's rules light (at least when compared to later editions), in fact so rules light that each campaign is different as each group interprets the rules somewhat differently. Some consider this a failing, I don't. (I consider Holmes Basic to just be an introductory form of OD&D and am therefore not ranking it separately).
#3: B/X. A good first attempt to rewrite OD&D to make it clearer and easier to learn. It's the best version of D&D in under 150 pages.
#4: 1e. Advanced Dungeons & Dragons tried to codify the game far too much for me. TSR at the time seemed to be interested in standardizing rules to make it easier to sell modules and because Gygax (at the time) seems to be very interested in (chess-like) tournament play where there could be player ratings, masters, grandmasters, etc. Neither of these interested me much. My games never really used AD&D 1e as TSR intended it, we just added what we liked from the 1e rules to our ongoing OD&D game. However, as time went on we used the AD&D books more and the OD&D books less.
#5: 2e. The second edition of Advanced Dungeons & Dragons seemed "meh" to me. It scarified the flavor of 1e and sanitized the game of features that TSR was afraid would upset the fundies -- like demons and devils. However, it was basically compatible with 1e and had some great settings (although the adventures that went with them were generally far too railroady) and perhaps the best monster descriptions of any version of D&D.
Note: There is a major break between #5 and #6. I enjoyed playing the above versions of D&D and would join in another game of any of them with a decent GM today. That's not really true of the following versions. The major reason why I would not volunteer to play the versions below is that combat takes far too long. I like the average combat to be over in 5 to 15 minutes of real world time (with occasional "boss" combats taking 5 to 10 minutes longer) -- and do not want to use minis/counters and battlemats.
#6: 2e with Player's Option books. The Player's option books was where D&D turned really sour for me. Complex character builds and detailed and time-eating tactical combat with minis and battle mats do not fall in the "enjoyable and fun" column for me. The fact that the Player's Option material often was simply broken in play did not help.
#7: 3e. 3.0 was basically a cleaned up version of 2e with the Player's Option books. It ranks below the original because a number of other changes were made that really hurt the game. Minor things mostly, like allowing spell casters to make concentration checks to avoid losing the spell they were casting if they were hit in combat and saving throws that scaled with the level of the caster -- changes that made spell casters far too powerful in practice. But the worst change of all was the idea of "rules mastery" where options that looked good at a glance but were really bad were put in the game to encourage players to study and master the rules. Putting "gotcha rules" in the game to zap casual players and favor hardcore players is a stupid idea that only helps narrow the pool of players while increasing the pool of "rules lawyers."
#8: 3.5e. This version made lots of minor changes to the rules (and to lots of spells), many of which made the game much less compatible with previous versions of the game in ways that one had to carefully study the rules to notice. This version also made it much harder to play without minis and battlemats. Combat still took forever. It was a haven for rules lawyers, especially if one used more than just the core books.
#9: 4e. To me, 4e seems to be a completely different game from any of the above versions that just happens to be called "Dungeons & Dragons" and have a fantasy theme. The designers decided that combat was the main "fun" thing in the game and so centered the game on carefully balanced combat encounters (that take far too long to play out) and discouraged wasting too much play time on the "non-fun to them" stuff between combat encounters. Character classes were completely redone to the point where the names are the same but that actual classes may not have much in common with classes of the same name in previous versions. Classes are carefully balanced to all be effective in combat at all times -- at the expense of removing most non-combat abilities from classes. With no "less combat effective" classes, there are no classes for players who do not consider combat the most fun thing in the game. Magic was nerfed to make it easier to balance combat encounters. Treasure became something players pick (or at least GMs are advised to give them what they want). Monsters were completely redone as combat fodder only. Etc. While a good number of people think this is the best version of D&D ever, I think it is the worst so far. I've played it a few times, but would never play it again unless forced to do so a gunpoint. It's just not any fun for me.