Day #18’s question was “Which RPG have you played the most in your life?”
This has to be Dungeons & Dragons 3rd Edition (3.0, not 3.5), primarily because of a long running campaign that was started in that system and no-one wanted to change or update things to newer editions or other systems.
I can’t say I’m a fan of the system. It has too heavy a focus on simulationism for my tastes, and it was full of vaguely written rules that led to lots of confusion. D&D, for me, also suffers from the “tyranny of choice” - too many options, especially with extra sourcebooks, made for decision paralysis or just rulebook hell having to refer to multiple different things all the time. Because the rules are relatively complex and crunchy, and some things would introduce entirely new rules or special case caveats to core rules, I found the game a bit frustrating at times. Much of the same issues I have with it still exist in 3.5 and Pathfinder, which is why I don’t generally find those game enjoyable. 5th edition is much improved though, and a much nicer, slimline version of D&D I can enjoy a lot more than previous editions.
Apart from D&D3, which really wins only due to a single, very long campaign, I guess I’ve probably played World of Darkness the most (if you group all their offerings into one - I’ve played pure WoD, Innocents, Werewolf, Vampire).
Then again, most of those games were short lived, so perhaps the Starwars D6 system wins out as a system I’ve played most. I’ve run a number of sessions of custom hacks, and played Middle Earth-based games, Starwars games, and others all using that system.
As for settings I’ve played in the most, that’d have to be the Titan setting, from Fighting Fantasy. The long D&D game I played was set there, plus I’ve played a couple of other things in it too.