Neonchameleon
Legend
At the risk of playing the grognard card, I'd have to say that original Fiend Folio is the book which most fired up my imagination; all of that wonderfully dark art by Russ Nicholson, and the whole enterprise bursting with the creativity of every writer in the UK ready to make their mark. Warlock of Firetop Mountain followed soon after, and 2000 AD was in its heyday; must have been something in the water. Or the Channel, rather.
That, and it had a flying vampire head trailing her guts below her.
M.
Oh, I have no doubt that it fired your imagination - and from that perspective (which is possibly the most important) was a huge success. And I don't mean to denigrate that. Also the Fiend Folio is a very nice book (there's a reason I have a copy) and really stood out from the other 1e monster books.
But I really think that a lot of the liking for older monster books is rose tinted spectacles. The 2e Monstrous Manual was (contrary to Steely Dan's assertions) massively better than the 1e Monster Manual and, as such, stood out like a diamond in the rough. It was almost a revelation. Monster Vault, as I believe several of us have demonstrated, has more and better fluff than the 2e Monstrous Manual. But while the Monstrous Manual left the 1e Monster Manual in the dust, Monster Vault is only a bit better than the Monstrous Manual - and it's a lot harder to inspire a 33 year old adult than the same person as a 13 year old kid.
As far as monster books being revalations go, the only one in the same league as the 2e Monstrous Manual (or even the 1e Fiend Folio) that I'm aware of is the 4e Monster Manual 1. And it was a revelation by D&D standards not because the fluff was inspiring, but because the statblocks were. @pmerton thinks that the fluff was good - and honestly I don't find it bad. But it's the representations where the rubber meets the road (i.e. in play) that leave any previous offerings in the dust or at least promise to. The 4e MM1 isn't bedside reading in the way the Monstrous Manual is - but it's incredibly inspiring to most people with a tactically oriented mind in my opinion and many kinaesthetic learners - this dichotomy is why it is so controversial. (I happen to fit both 'kinaesthetic learner' and 'tactician'). 4e Monster Vault offers both superb bedside reading fluff and statblocks that are what the MM1 promises but with more polish and that almost invariably deliver, but the bedside reading isn't the revalation the 2e Monstrous Manual was and the statblocks are mostly polished versions of the 4e Monster Manual 1.