Out of what I've seen my least favorite MM so far has to be the Ravenloft Denizens of Darkness. I mean, come on! For the steep price they're asking I'd expect a little more meat. Color art isn't essential, but if I'm paying a buck for less than ten pages if it ain't there _everything else_ better be... and it wasn't. The number of monsters alone was pitiful. Though prolly worth $16 as a softbound.
For my favorite, I'd say the 3e MM covers so many bases that it's got to rank pretty high, but I suspect the Tome of Horrors will be even better. (When does that thing come out, anyway? I've got most of the stuff that'll be in it printed out and in a binder, but I'd like to thin it out...)
As for the Fiend Folio- lemme give you scoffers a list of some of the great things from that book:
-slaadi, githyanki, githzerai, sons of Kyuss, thoqqua, Lolth, mustard jellies, algoids, adherers, magnesium spirits, necrophidii, scarecrows, dune stalkers, stunjellies, sussurus, disenchanters... lots and lots of goodies (imho). Not enough for ya? How about caterwauls, caryatid columns, elemental princes of evil, crypt things, yellow musk creepers and zombies, xvarts, penanggelans, apparitions and acherai?
Granted, the FF was the home of some of ad&d's biggest dogs, like the nilbog and the clubnek, but there's a lot of great stuff in there. Give it another chance. Convert something weird and use the FF pic for it for a trippy, dirty-feeling monster.