Message-bord art discussions are always fraught with peril, because of the subjectivity. I personally like a lot of the 70s-style art; for whatever reason, it pushes my buttons and fires my imagination.
Here's some specific C&C pieces that I like (Players Handbook pg references are to the 2nd printing) Many of these are Peters -- some are not:
PH, pg. 10. I like this one because it presents a classic adventuring party. I love the trees/darkness they're heading towards. They're clearly off into the unknown.
PH, Ranger pin-up, pg 12. I like this one because the PC looks cool, and looks like a Ranger, to me (love the beard). I also like the clean black-and-white lines, with just enough shading/grey tones.
PH, Wizard pin-up, pg 21. This one has more shading/grey than I usually like, but the subject is excellent. The pipe and the floppy hat really kick this one up a notch, for me.
PH, Illusionist, pg 22. Peter draws very attractive women. I won't list a bunch of them, but I like this one, as well as most of the others (I have a soft spot in this direction). Clean lines, again, too.
M&T, Doppleganger, pg 19. Just twisted. The dark tone suits the subject.
M&T, Dragon attack, pg 20. Clean, dynamic without going into action overload.
M&T, Ettercap, pg 33. Strong contrasts. Evil looking.
M&T, Lich, pg 54. Fires my imagination. Love the background.
M&T, Mummy, pg 60. Maybe I just like undead. Definitely like the style.
M&T, Owlbear, pg 65. The style suits this subject (hair/feathers). I get a sense of menace and impending action. There's going to be one hell of a fight in a second.
M&T, Satyr, pg 72. I like the "looking from the shadows thing." Tells a story.
M&T, Shadow, pg 72. Much like the Doppleganger, the dark tones are perfect. Like the background, too.
M&T, Poisons, pg 126.
I also think the maps in the Eastmark folio are beautiful. (Peter does a great job on maps -- his work on XXXI is excellent, too.)