I think late 1e starting around 1986 through to around 1995 (give or take a year to either side) marks the golden age of D&D art. This is the era of Keith Parkinson, Larry Elmore, Jeff Easley, Clyde Caldwell, Fred Fields, and Daniel Horne, and also the era when they made the critical decision to give Brom the entire Dark Sun line and Toni Diterlizzi the entire Planescape line, which so much enhanced and even created the feel of those settings.
Earlier TSR art is iconic, and some of it is quite good, but not quite as polished as later eras.
WotC art direction has IMNSHO been largely terrible, and this is set by the way WotC has handled the art for the MtG line which has also fallen off from its early days where you felt like you were collecting miniture fine art. While WotC does correctly understand as TSR did that a setting needs to have a consistent feel across all of its art, the WotC approach is so heavy handed that it drives all the creativity out of the art and tends to stifle the artist. The art directors at WotC also have for some good reason come to prefer clarity and cost over almost anything else when managing artwork, and while this is somewhat understandable when it comes to playing pieces that have art on them it doesn't make for good art. Instead, it tends to prefer a comic book style to the detriment of the look of both their card game and even more so their RPG. So much of WotC art direction is focused on illustration, that it never manages to achieve actual art.
I disliked most of 3e art and hated 4e art, but it wasn't like that all the art in those periods was universally bad. For example, I would have far preferred if the 4e product line was done in the style of Eva Widermann over that of fan favorite Wayne Reynolds. Likewise Todd Lockwood did good work that I would have liked to have seen more of.
That said, 5e feels like in art (as in many other ways) a return to form, with the new art often so good that it would not feel out of place in the middle of TSR's golden era and likely would have in cases been hailed as a classic.