I'm more of a fan of the earlier SR editions (1-3), but it's probably because of some of the nuances that didn't go to 4th (cursed nostalgia). That and I had every book until 4th hit the shelves...
Ars Magica is fun and encourages creativity and roleplaying (Magic centric system).
I've only played pre-CBT Battletech, but with a few house rules it played out fine (who doesn't like giant robots fighting it out?!).
Cyberpunk is like SR without the magic and amped up craziness and cyber-ness...
Darksword Adventures was a one book system by Weis and Hickman associated with the named trilogy. Roleplay and creativity heavy! It operates on either percentage dice or a paper-rock-scissors related, which worked better when we were out hicking (we adopted this conflict resolution style to other systems when we out and about without dice).
Deadlands, is cool if you're looking for 1800's style play. I haven't played it yet, but I read it through and it has a different feel than the normal systems.
D&D 3-4 (e) is for the MMO generation, because they wanted to ultimately put the games online. AD&D 1-2 (e) was more roleplaying heavy and it has that classic RPG feel (good luck on finding them though - Goblinoid Games makes a clone that's closer to 1st (e) called Labyrinth Lord). 1st (e) was the only game where players had trouble making it through character creation with that optional 0th lvl start, where your characters become their class and stats piece by piece through roleplaying.
Earthdawn is SR in a fantasy setting, minus the tech.
Eclipse Phase is pure scifi with a very streamlined system, you'll see some of the SR influence in it.
GURPS is like playing the tabletop version of Fallout 3, there's even a module for it if you want to go that route. It's like the percentage system of the d20 system, but older (I wouldn't be surprised if WotC was inspired by it when they put together d20).
I've played everything Palladium put out, but out of everything they have I always went back to Rifts. Whenever we had a player that wanted to play a certain character in a particular system we could almost always convince them to play it in Rifts, where it could make sense that they would suddenly appear.
I'm very pro WEG Starwars (d6). It always played out smoother than d20 and it'd be easier for SR players since they have plenty of the dice. Starwars is a open license right now since Lucas yanked it from WotC, because of where they were going with Saga (my brothers a moderator on the WEG SW site, so I know way to much of what's going on).
Warhammer Fantasy is an RPG, not wargame. It's set in the named setting and it plays out kinda like Earthdawn with a Warhammer feel (they aren't making it anymore). There is also the 40K RPG, which I've read but haven't played. If you like the universe from the books or the VG's then head in full bore.
WoD and Exalted are for college players that have that "middle of the night out of your minds" moments, but you have to do your homework because they don't clearly chart anything (or they didn't before they reboot their worlds).
My group gathered together our favorite things from every system and universe and ran with crazy home-brewed fun. Try it, you might not go back!