Mechwarrior - I've played it a lot, but always as the GM, rarely as the player. The Battletech universe is probably my favourtiest RPG universe ever, one I've been following since the dawn of time. I'd especially like to play under the current A Time Of War rules. 2d6 4 lif
Shadowrun - I've allways loved the setting but never had a chance to play it beyond a few high-school games aeons ago. It's one with a lot of potential and some great crazy/wild/etc ideas that I'm just itching to play with.
Non-d20 Star Wars - Paradox; I love the Star Wars universe, I loathe the d20 system. However, since I've never met someone who plays WEG Star Wars and I've never seen a full conversion to any other system, I will remain in perpetually frustrated land.
I'm with Kayla on most of this... except the WEG Star Wars. Force Users were so incredibly broken in WEG Star Wars that it wasn't worth playing.
I have played some great Mechwarrior campaigns, but unfortunately something always distracted us when it started getting really good, and we'd end up moving onto something else.
I had a ton of fun with Shadowrun, when crashing at a friend's place back in the late 90s. He and his wife had pretty much ALL of the Shadowrun books, and we ended up playing well over 400 hours of Shadowrun in a 4-player group (plus GM). Unfortunately, that ended when my friend was shipped overseas to Germany for six months (and he was the GM).
Another game I used to play a lot was Legend of the Five Rings (first edition). The Second Edition of the game was "meh", and I was not happy with the 3E/d20 variant (though I liked having the option for Forgotten Realms campaigns). It has/had a great character creation and combat/dice-system to it, a very simple method somewhat similar to oWoD's success/failure system.