Overall, I have to give the nod to AD%D 2nd Edition for Best Art.
Perhaps not in the interiors of all the books (although some of the B&W illustrations in those were excellent), but with the quality of the book covers and Dragon/Dungeon cover illustrations, 2e pushed ahead by, IMO, a wide margin.
3e has had some very good interior art, and none of the 3e art is as bad as the worst 2e (much less 1e/OD&D) stuff, but the best just doesn't come close.
Personally, since the 'realistic' art is of a much lower caliber than it used to be, I would rather see a more anime or comic style. d20 Modern did a good job with many of its illustrations - clean lines, bold colors, a very distinctive feel. Moondog as the bodyguard especially leaps to mind as a cool illustration in a non-realistic style. Something about most of the D&D 3e art just feels bland, IMO.
Also, d20 Modern's iconics > D&D's iconics in form and function. They actually improve, illustrating the game system in the process. Seeing how Russel looks as a Strong Hero, a Soldier or a Shadow Slayer shows new players how characters can develop in d20 Modern.