Regretfully, I no longer seem to have the affinity to quickly pick up and understand diverse rulesets these days. In my younger days, I ranged far and wide:
MERP - I still remember some crazy NPC chasing the party yelling "Bola?!? Bola? You want bola? I give you bola!" as they ran in terror. This morphed into Rolemaster, which then killed the campaign under too many rules
Swordbearer - we never got further than character generation regretfully, but I did use the Dwarven Halls adventure in AD&D.
Star Ace / Timemaster / Chill / Sandman - I ran them all, but I do vividly remember a player's space fighter being destroyed around him, and he was left floating alive in space during the battle.
Traveller - more character generation than anything else, but I did run a few one or two shots - the planned campaign never seemed to materialize.
Warhammer Fantasy Roleplay - we had a campaign that lasted for months, before the party's spell caster inadvertently cast a fireball with himself as ground zero. Did I mention he carried like a dozen glass flasks of oil?
Paranoia - OMFG, but this was a blast to run. Killing players without remorse - Here come the clones! Amazing in small doses.
Top Secret - fun for one shots, there was little interest in playing this more than a handful of times.
Gamma World 1e - I think we played a 2nd session, but I'm not sure. There just wasnt much love for it in my group. First game I bought with my own money as a teenager (during my first trip to the Complete Strategist in Manhattan).
Star Trek (FASA) - tried to use it in conjunction with the Command Deck game for it - we never got to a second session.
Mechanoids - a single session
Palladium Fantasy - we played the digest sized published adventure with the red cover - and always talked about going back to it but never did.
Call of Cthulhu - a handful of one-shots, usually in October
Runequest - I got the 3rd edition of the rules for Christmas one year and never understood the sorcery rules. Grabbed the 2e rules, Pavis and Big Rubble at a con at Columbia University, and ran 2e for a summer and then some as a sandbox type setting. Fun times.
Conan (TSR) - ran a few sessions before we all decided we preferred D&D by far. Wasn't even fun as a change of pace.
GURPS - always wanted to run it more then I ever got the chance to, two campaigns ended in the first session with TPKs. Ah well.
Elric - one session. It deserved more, but I dont think any of us had a handle on the setting and expectation at that time.
Champions - this was the boxed set. Balancing the powers got way too confusing. I've never enjoyed supers games much anyway.
Spacemaster - I ran this for a two player group for the better part of a year. Still don't think I knew half the rules, but it didn't keep us from playing and having fun.
Cyborg Commando - The Complete Guide to RPGs gives this 3 1/2 * - I give it 4 shits. My God, but this was awful. I can't believe I made my group even try to play it, but we did. I'm still trying to live it down.
The Morrow Project - one of my players really wanted to play this, so he gave me his rules. We got as far as character generation. Ah well.
Pendragon - we tried, really we did. Not enough stuff to kill for my old group ;)
Toon - I ran a few sessions when the group was short a player or two. It was a fun change of pace.
Boot Hill - we ran a session that seemed more like a single unit miniatures war game. No one wanted a second bite of the apple.
The above is a fairly complete list - I suspect there are a few I've inadvertently missed. Still amazes me that I had a decent handle on such a diverse selection of rules.