I liked 2nd Edition with the Skills and Powers books a lot, custom designing character classes and detailed critical hit effect tables...
...but honestly, 3rd Edition does it for me. It's much easier to learn than 2nd, and integrate new players into. At the same time, it was easy for older D&D players to adapt. The system had it's downfalls, but it was pretty openended and fun. The prestige classes concept appealed to me.
2nd Edition before Skills and Powers suffers a lot of the same criticism I point at 1st edition below.
Now to discuss that which I have only limited experience of.
1st Edition -
This edition as I understand it had very strict rules, very detailed and obscure rules, and was not only hard to learn and master, but also limited character options, with many restrictions on race and class/multiclass combinations.
4th Edition has some interesting concepts but I'm not buying a full set of books again for a few years. Also it seems a very different game and that may be good, may be bad.