I'll take this to mean non-D20, otherwise Conan D20 would be right up there in the list.
Here is my list, in order of Awesome as All Hell (
#1 ) to Still Freaking Awesome, But Not as Much as the Other Ones (
#4 ):
#1 Warhammer FRP - Gritty, evocative, sometimes downright scary, with deadly mechanics that make you feel sorry for your character in much the same way Call of Cthulhu does.
The background world is just amazing...in a nutshell, WAAAAAAAAGGGGGHHH!!!
#2 Earthdawn - Another one that can be scary, due to the Horrors being, well, horrific. But here you truly have a mix between CoC and D&D. Super tough monsters (not just horrors) and environment meet truly capable characters, and the combination is enough to excite even the dullest person.
The world of Earthdawn also has an incredible backstory and it actually allows you to set the game in very, VERY different situations. I particularly enjoy putting my group in the position of being some of the first to leave the Kaers. The wonders of Rediscovery abound.
#3 Runequest - Rules that make sense (it's based on BRP, just like CoC) and are super easy to handle. Several options of campaign settings to choose from (Glorantha, Hawkmoon, Elric, Lankhmar, Slaine) that can get you vastly different ways to enjoy the game.
Also, it's got an incredible number of supplements detailing Glorantha, and even a bunch for the other settings as well, and they can be had really cheap on Ebay, something which does NOT hold true for the other two options.
#4 Ars Magica - The most creative game sessions I've ever seen were likely Ars Magica ones. The freedom of the magical system, together with the obviously appealing setting of Magical Europe (everything present in history, with the added caveat that all magical tales/superstitions/etc are TRUE) make this a true gem.
The game is as political as it can get, and RPing is vastly more important than stats on a sheet of paper.