Totally agreed on the FF, though there's a lot of chaff in with the wheat. The weirdness quotient and originality is high, and of course the art adds wonderful flavor.
The 1977 MM has a lot of original stuff, but is also stuffed with junk in places (like the excessive dinosaur list), and has dubious stats and editing, stuff like the damage for a lot of creatures seeming to be written for OD&D rather than AD&D, and notoriously unclear/unexplained stuff like how demonic spell-like abilities are meant to work.
I think GAS has a point about the 1993 Monstrous Manual in terms of it being a well-curated list of core monsters with fewer issues than the 1977 one.