My favorite D&D artists all seem to hail from the 2E era (Merric mentioned these above - Caldwell, Elmore, Easley), and many of my favorite pieces come from Dragonlance. On the other hand, I started playing during the 2E era (first the "Classic D&D" boxed set, then the 2E PHB - and these both had great cover illos) and DL was my favorite setting. Obviously, nostalgia is a factor here, but I also think it was partially the art that attracted me so much to the game. Some posters above have criticized the "technically proficient" 2E art as "stiff". Frankly, I don't see anything stiff about it, and technical proficiency may be underrated. The 2E art often featured smooth, rounded edges and surfaces, and was probably more 'realistic' (given that this is fantasy art) than either 1E art (which I enjoy, even though I don't think it's as good) or 3E art, which seems to be heavily influenced by computer games, anime, manga, and comics. For me, the sheer quality of the art allowed me to really immerse myself in the setting and imagine it more vividly. I don't see too much of that in 1E art, and very little of it in the 3E art, either. If there is any artist working on 3E who is as good as Jeff Easley, I haven't seen his or her work, yet. Exception: there is some darned good art in Arcana Evolved and in Ptolus, but these are not WotC.
EDIT: I just realized that a lot of the 2E art (like the original cover for the 2E PHB) was more 'historical' than the current "dungeonpunk" aesthetic. In 2E, I ran a lot of historical fantasy games using the green HR series. That said, I actually rather like the whole tattoos and buckles thing, and it has probably influenced the kinds of games that I run in 3E. 3E is more explicitly fantastic - what with two-bladed swords and all. In 2E there was a tendency toward historical accuracy that is just not there anymore. If I were going to run a historical game now I would probably use a variant of d20 modern or even Storyteller.
Come to think of it, my favorite D&D artist of all time has got to be Tony diTerlizzi, and I never even played Planescape - I bought the books just for his art. In fact, yesterday I was at my local independent bookstore doing some Christmas shopping for my daughter, and I found this beautifully illustrated children's book. Just looking at the art, I knew I (er... my daughter...) just had to have it. Guess who the artist turned out to be? DiTerlizzi! That put a big smile on my face.
Everything above is IMHO and YMMV. De gustibus non est disputandem and all that. I was just surprised that no one had really come out in favor of 2E art, and I thought I should speak up about it.