What a cool thread! It's hard to say, though -- most of the monsters I didn't like I still don't, and most of those I did, I still do. That said, there were some products put out that changed my opinion of some creatures.
For instance, I never liked beholders or mind-flayers much from 1e, and I didn't see anything in the early books of 3e to remove that stereotype. Lords of Madness has definitely given them a new coolness, though. In fact, aberrations in general are much cooler to me than they used to be, because they're not just silly combinations of weird animals anymore. Well, a lot of them are, but there seems to be a better emphasis on Lovecraftian Far Realms type aberrations lately, and it has really increased their coolness.
I was always ambivalent about dragons; they seem cool enough on the face of it, but I didn't much like the chromatic and metallic varieties, and the general weakness of dragons over all. The Monsternomicon really changed my tune about dragons though -- those three dragons listed above, as individuals, no less, are too rockin' to be believed.
I always thought the displacer beast was pretty stupid, but the illustration in d20 Modern of one eating a Santa Claus was really cool.
Orcs are more than just mooks now, and thanks (probably to Warcraft) they're more than just evil parodies too. I heartily approve.