Greatest non-D&D/d20 fantasy RPG adventures?

[Glyfair]

The Enemy Within campaign for Warhammer Fantasy RPG.

I considered this as part of my list. Although I've heard many good things about it, I decided not to put it on my list. The reason is because I played in it (albeit, briefly), but can't remember a single thing about it.

That probably would have changed if I had been able to stick with it longer (I think I managed 2 sessions). Still, it does have a good reputation.

 

[Khairn]

My all-time favorite is

Dark Space by Monte Cook. This Rolemaster/Spacemaster module presented a collection of planets in a restricted volume of space, all being threatened by a horror that gave me and my players nightmares.

Aside from other ICE modules tht have already been mentioned I would say that Montreal by Night for Vampire would be my next favorite.