I'm not a big fan of 1E D&D art as it was too cartoony, while most modern D&D fantasy art looks too computer generated and fake, and in the continual quest to be "diverse" it lacks the strong European medieval flavour that just tickles my fancy (and is a reason I don't like D&D Kung-Fu monks). Pathfinder rulebooks are particularly jarring in this regard - though I imagine the images are appropriate for the campaign setting.
As an example, if you have the 5E PHB the illustration for the Soldier background looks like a Samurai to me. It just seems a little jarring - I'd have expected a bog standard soldier. Similarly the Fighter image is some sort of tribal warrior - who could be a ranger for all I know given his relatively light armour, the Paladin is inexplicably a half-orc with an axe and heavy armour and the rogue looks like an assassin.
Probably my favourite pieces are Elmore's cover for the Companion Set of D&D - which I've never owned, going from Red Basic + Immortals, to the Big Black Dragon box and then directly to the Rules Cyclopedia. Remember I'm the guy who started gaming - with Advanced Heroquest. I've always been hardcore.
The image was used on one of the D&D Novels, the Tainted Sword, Book 1 of the Penhaligon Trilogy by D.J. Heinrich, which is a surprisingly mature book and is easily the best in the trilogy. That's where I encountered this lovely artwork that represents aging Sir Flainn Flinn fighting the evil dragon Verdilith:-
|There's your Dragon, now where's the Dungeon?|
Of course there is the old classic, though when I looked in awe at the adverts in grainy imported comics I could never tell which way that warrior was facing. It still looks like an odd pose, and the armour is being worn by the gentlemen badly.
|That Barbarian is going to have one heck of a back problem.|
But the other contender is the cover to the original Advanced Dungeons & Dragons 2E PHB. There's just something very cool about that fighter on horseback and the shadowy party of riders behind him that could be your characters. Very restrained, these days Marketing would probably send it back with anote saying, "Add Dungeons, Dragons and big monsters to the cover" which is clearly how we got the 5E covers to... well.. any of their products.
|Onward to High Adventure (tm).|
That said there's been a ton of amazing artwork for things like LotFP, Shadows of Esteren, the various Song of Ice and Fire RPGs and Warhammer.