Most of them, if we're honest. The criteria here are a little bit difficult, because the "again" part dashes some of my hopes with other games.
If I have to be selective, though, I'll limit myself to three (and not my own, because that's cheap): Earthdawn, Shadowrun, and Twilight: 2000.
(questions and image from autocratik.blogspot.com)
I only ever got through the basics of starting a campaign in Earthdawn before it petered out, and I've always regretted not running through at least a few introductory sessions to get a feel for the game. My group just didn't have the discipline to build characters that complex without assistance, and I didn't have the time or expertise as a GM to help them through the process.
At the same time, it's a fantastic system and I really want to finally find a lasting game of it.
However, that's partly disqualified because I never really got into the play; we got the initial setup mostly done (at least a few of us, not everyone), and then just never played.
That leaves Shadowrun. I have never played my favorite edition of Shadowrun as a player beyond one-shots, and that makes me a little disappointed. I really want to have a chance to play in a Third Edition game at some point. However, I don't particularly feel like playing in the more recent editions, which are the only ones I've ever really played in.
Going back to the disqualification thing, however, I technically ran Shadowrun, but I didn't play it as a player, so we'll roll over into my next most wanted, Twilight: 2000.
Twilight: 2000 is from a different era, and it stands out because of the fact that it's not built like a lot of modern games. It takes a very ambivalent, simulationist approach to whether you live or die, and it has an element of chance and survival gameplay that you just don't see in most games.
As someone who appreciates the flexibility and opportunity such a system provides, and as someone who liked the old Traveller rulesets, I would love to have a chance to play it again, and it's free from disqualifications because I've actually played it, not just run it.
So, there you go. A needlessly complex answer to a simple question, but hopefully an interesting one.