I'm really surprised nobody's mentioned Lakofka's
"Bone Hill" yet - whenever anyone asks how to design an adventure that's where I point them. Great stuff in it, and enough ways to approach it all that it's never the same twice. It's designed for low-ish levels but it's dirt simple to beef it up if needed.
"Forgotten Temple of Tharizdun" is another favourite to run - it's simple to DM, the players always love bashing Giants, and it's the sort of thing that's always easy to drop into any campaign.
"Quest for the Heartstone" (at least I think that's what it's called) is another one I've run a few times and had lots of fun with. Make sure you use the pre-gen encounters on the journey to the place as well, a few of them are priceless!
I think I'd have a much better opinion of
"Forge of Fury" if I'd ever been able to run it all the way through - the one time I tried, the party bailed after only doing about half of it. (in fairness, they'd completed their mission; they just didn't stick around and explore the rest of the place) But the half I ran was pretty good.
My all-time favourite, however, is
"Nakki-Nakk", a friend's homebrew adventure* - I've run it twice now and would again in a heartbeat if I only had any players who hadn't already seen it!
* - this was also the very first adventure I ever played in; I didn't know what to make of it at the time, and only much later looking at it from a design (and DMing) perspective did I realize how good it really is. Players I've run in it since universally loved it.
Lanefan