AO
Difficulty: 4/5 (hard but not impossible)
Graphics: 4/5 (the backgrounds are very good, and with the characters, it's even better)
Enjoyable: 4/5 (good story, good gameplay... and the cutscenes!)
Overall: 4/5
Hardest level: Maybe the Paramonian Temple. I hate those swinging rocks! And the bats too.
AE
Difficulty: 4/5 (easier than AO, with the QuickSave and stuff)
Graphics: 4/5 (not much difference from AO, but still great)
Enjoyable: 5/5 (still good story and gameplay... And the possessing of wildlife and Glukkons!
)
Overall: 4/5
Hardest level: Necrum with those Fleeches. At least I'm glad when get past them.
MO
Difficulty: 3/5 (easier than the last two games... lots easier)
Graphics: 5/5 (the 3D environment looks very good)
Enjoyable: 4/5 (sometimes a bit boring, like Fuzzle Guy told earlier, No Muds Land or the Slog Huts, for example)
Overall: 4/5
Hardest level: I have a German version, so I don't know every single level name, but I think it was Loading Bay, the first level in Vykkers Labs. That place where you must jump up with Munch and get rid of all Vykkers and Interns. That's a hard one.