By way of Tim Brannan at The Other Side, the seven roleplaying games I've most played or run. In no particular order -
1. Dungeons & Dragons: It's hard to avoid playing the World's Most Popular Roleplaying Game, though it was far from my favorite, and like many others, it was my first - the Holmes blue box, then 1e AD&D, then 3e. I never played 2e, 3.5e, or 4e, and I'm unlikely to ever pick up D&D? Next!. D&D is the game I play because, well, everyone plays D&D at some point; recently, that's been one or two weekends a year, and that's about all the D&D I want or need. That's not to say that I don't care for D&D itself - there are actually a number of things I really like about the game - but rather that I'm just not a big fantasy guy. If I ever play this again regularly, it will be with my kids, and it will be a swashbuckling & sorcery campaign.
2. Metamorphosis Alpha: MA remains the perfect dungeon crawl roleplaying game, in my experience, way better than anything I ever did with D&D. Thanks to my parents, I grew up reading Robert A. Heinlein, and that included Orphans of the Sky, so the basic premise of MA resonated with me immediately. The real problem was that I could never find anyone else who was as deeply interested in the game as I was, so I was only able to run it sporadically. I did import much of the technology into my D&D games, however, 'cause sci-fantasy was more palatable to me than the bog-standard pseudo-medieval stuff most people were playing.
3. Boot Hill: I loved Westerns from the time I was a boy, thanks to my dad's influence - we spent a spring break when I was about eight visiting ghost towns and exploring old mines in the Mojave Desert - so I got my hands on 2e BH as soon as it was available. I think I like BH better now than I did then - and that's sayin' somethin', 'cause I played an awful lot of BH - because with more than three decades of experience behind me, I appreciate the lean rules of the game even more now than I did as a teenager. Two of my all-time favorite modules, for any roleplaying game, Mad Mesa and Burned Bush Wells, were published for BH.
4. Top Secret: Pretty much everything I run turns into a spy caper at some point, so TS was right in my wheelhouse. Beyond the genre, however, I really like a number of the rules in TS, such as secondary attribute scores and the way contacts work in the game - it's said by some that the rules tell you what a game is 'supposed to be' about, and together the contact rules and the deadly combat rules tell you that this is a game about investigation and manipulation rather than going in guns a-blazin'. The problem is, the first few modules kinda failed to sell that: Operation: Rapidstrike! and Lady in Distress were both commando missions used for convention tournament play, and the woefully misunderstood Operation: Sprechenhaltestelle, which came packaged in the box set, comes off like a dungeon crawl with Uzis rather than the excellent espionage sandbox it really is. TS really hit its stride with Operation: Fastpass, another of my favorite modules for any system. TS was really well supported, with a ton of Dragon articles and the Top Secret Companion. I'd play this again in a heartbeat.
5. Traveller: Years ago, I wrote that of all the roleplaying games I played, I had more good memories of playing Traveller than any other, and I think to some extent that still holds true. This is the only game where I play the house setting; I love Charted Space and the Third Imperium, in all their flat-space vastness. Traveller is the game that initiated my personal 'old school renaissance' - I was considering putting together a d20 Modern supplement similar to 1001 Characters, and when I was flipping through the pages, I thought, Wow, this really is a great game. A week or so later I had the reprints in my hands. Running the captain of a free trader looking to become a merchant prince is still one of my favorite roleplaying experiences.
6. d20 Modern: With my inclination toward historical and contemporary gaming in the real-world, this was my go-to game for many years, the only generic system I ever really liked.
7. Flashing Blades: *looks around the page* Well, yeah.