From Tim Brannan's Other Side by way of Black Vulmea's Really Bad Eggs, comes the idea of posting about the 7 adventure games you have run or played the most. Here's my list, in rough order of amount of time spent playing or running them:
1) Dungeons and Dragons
Especially if you count d20 variants. I've been playing some version of D&D off and on since 1993. Started with the Rules Cyclopedia, moved to 2e, and then abandoned it in the mid-90's only to pick it back up around 2003 and play it more or less continuously until early 2010, when the guys in my group decided they hated 4e and we collectively decided to try other systems. And even then, in 2011 I started playing Swords and Wizardry, and in 2012, Microlite Iron Heartbreakers. Of the 21 or so years since I started playing, I have been playing some version of D&D for about 14 of them.
It's odd because though d20 is inarguably the roleplaying game system I am best at manipulating the mechanics of for personal and team advantage, the D&D family of systems has never been a favourite of mine. Swords and Wizardry and the other rules light variants / retroclones suit my purposes well enough to use for paedogogical purposes. I use them to train people in the basics of playing adventure games with the hope of eventually moving onto crunchier systems.
2) World of Darkness / Storyteller
I used to be much more fond of this family of games than I am now, though I do consider the games to have mostly gotten mechanically better over time. I played a lot of Mage: the Ascension online between 1998 to 2003, which was a gaming drought for me in the physical world, and a lot of Exalted online between 2000 and 2006 as a supplement to it. I also a ran really strange, very short-lived Hunter game when I was 17 that was based on the players playing "adult" versions of themselves (the characters were them imagined at age 25 or so). All told, I've playing Storyteller games for about nine years of my gaming life. Between 2010 and 2011, as what used to be my main gaming group moved away from playing D&D, we took up the new World of Darkness system, though most games were short run horror-themed mortals games. This is part of a general move on my part away from horror-themed games, and away from rules-light games towards crunchier systems.
3) WFRP / 40K
I calculated a year or so ago that if all my current WFRP 2e campaign obligations as both PC and referee are to be fulfilled, I will be playing campaigns of it until sometime in 2015. I started playing it online years ago, in 2005 IIRC, then moved to playing it in real life in 2007, and have been playing it with some group or another ever since. Since then, the only major interruptions in playing some session of WFRP 2e have been to play short runs of Rogue Trader and Deathwatch. I figure it to be about seven years of playing it in some form, typically in biweekly sessions or play-by-posts.
Of all the systems I play consistently, WFRP 2e is the one I like the most, though of course I have a few complaints about it. I like the power curve, I like the setting, I like mechanics for the most part. I wish the gear list was better, though it's pretty good. At the very least it should have the prices for more of the career trappings. I also find the specialties of some skills excessively narrow, though this isn't nearly as bad as in Dark Heresy. When I run WFRP 2e I tend to run it very differently than the people who are my main referees for it, but that's mainly a stylistic issue.
I can't really stand Dark Heresy in many ways, but I have played a ton of it, and I've run games of it, as well as Rogue Trader and Deathwatch. There are many, many elements of DH I would rewrite / have rewritten for my own games, to the extent that I'm almost playing a different game. Still, I love the setting, and the actual campaigns were great.
4) Heavy Gear
From 1994 to 1999 Heavy Gear was one of my go-to science fiction games, along with Alternity. One of the great things about the game was the incredible buy-in to the setting from the group I played with for most of my adolescence, at least partially due to the war game elements and the tie-in computer games. PCs went out and bought, and then read, setting books and supplementary material, and I could drop casual references to other elements of the setting in and rely on the PCs catching them. It was a golden era of young gaming.
The game where I got into the knife-fight at the table with one of the PCs was a Heavy Gear game in summer 1998. Superficially this probably seems like it coincides with the end of my RL gaming for about five years, but in actual practice we played for another six months or so (mostly Alternity at that point), and it was only at the tail end of 1998 that we ceased meeting regularly. When I reflect on Heavy Gear now, it's always with a bit of melancholy and nostalgia, and I feel like I can't really go back to this system and play it in a clear, adult way. Playing Heavy Gear basically ruined me for any other mecha game, and I've never gotten back into the genre in the way I did when I played it as my preferred game in my teenage years.
5) The BRP family (Call of Cthulhu, Openquest, Mongoose Runequest 2)
It's odd that the family of systems I like so much crops up so low on this list I suppose, but that's because I discovered it so late. I first played Call of Cthulhu at Giant Space Telescope Con in 2009, playing the sonar operator in Grace Under Pressure. It was also the first time I played a prewritten module. Upon discovering the then-newly-released Mongoose Runequest 2, and Openquest, these systems won my heart. I've played a bit of each, though nowhere near as much as D&D, obviously. I hope that over the course of the rest of my gaming life, this ratio will even out, or even shift in favour of BRP.
6) Teenage Mutant Ninja Turtles And Other Strangeness / Rifts / Heroes Unlimited / Recon
The first roleplaying game I ever played, from 1991 to 1994, off and on. I am a self-taught gamer who came to roleplaying through an adventure game that was not D&D, both of which are somewhat unusual as I understand it from hearing others discuss their backgrounds. TMNT was great, though the kids I mostly played it with were my toxic and horrible neighbours, who taught me at a very young age how terrible people can be in groups. I'm not saying my interest in group dynamics in adventure games from a set of experiences I had when I was eight, but that interest is definitely informed by them.
Heroes Unlimited and Rifts were grafts onto the same group, and while I am still nostalgically fond of the systems, I experience even less desire to play them. Of all these games, only Recon, which I played later, with the same adolescent group that I played Heavy Gear with, is one that I'd like to revisit. It wasn't the first military game I played, but it was the first one that was based on a real conflict and that emphasised the complexities of military activity, and that wasn't really heroic.
7) Everything Else
At this point, the list starts to break apart into a basically arbitrary selection. Any of Diaspora, Traveller, Burning Empires, Burning Wheel, Unknown Armies, FUDGE, Star Wars D6, Shadowrun, Nobilis, Stars Without Number or Alternity would qualify. I've probably spent a bit more time running Alternity or Unknown Armies than the others, but Traveller and Stars Without Number are systems that I use as supplements to other games I'm running (the world generation systems, in particular). Most of these are games that I've run or played in one-shots, or short series of games, or campaigns that failed to launch, or that I draw on as supplements or reference books. There are a couple that I would love to play more often than I do (Diaspora, Stars Without Number and Traveller) but can't fit into my available gaming time.