Mythras (formerly RuneQuest 6 and Mongoose's RuneQuest II).
This is the game that I've played (as opposed to GM'ed) the most in recent years. Highlights include: (a) playing a half-Melnibonean sorcerer in the Young Kingdoms (using MRQII) and (b) playing a Roman mystic-philosopher (in Mythic Britain). Among its many virtues, Mythras has (hands-down) the best combat system I’ve found in a RPG yet.
Call of Cthulhu.
I've run a fair amount of this over the years, both campaigns and one-shots. It is probably my favourite game to run overall.
Classic Dungeons & Dragons (including Basic/Expert D&D, AD&D, and related 'retro-clones' [e.g., Swords & Wizardry, OSRIC] and 'pseudo-clones' [especially Crypts & Things]).
I haven't played too much 'classic D&D' in recent years—except for a short AD&D campaign and a number of sessions of Crypts & Things (a game to which I contributed some rules)—but it remains a constant love for 30+ years.
Dungeons & Dragons 5th edition (especially as modified for Adventures in Middle-earth).
Finally, an 'in-print' version of 'D&D' that I enjoy! I'm currently running an AiME campaign. It's a pretty solid realization of Middle-earth.
Elric! (a.k.a. Stormbringer).
An old favourite (especially Elric!/SB5e). It’s a fantasy version of BRP (Basic Role-playing) that is easy to run and has a lot of flavour. While I think that the MRQII version of the setting (now out of print, alas) does a better job in realizing the Moorcock’s multiverse, I have fond memories of this game.
Middle-earth Role-playing (MERP).
I haven't played this game in years (the last time, I think, was 1999-2000), but I cherish my ICE collection of MERP adventures, campaign guides, and (especially) maps. Indeed, I am using some MERP material in my current AiME campaign. I ran my first 'real' (i.e., coherent) RPG campaign using MERP while in high-school, and still look back fondly on it. To this day I enjoy reading the critical charts.
There certainly are other games that I’ve played and enjoyed over the years (e.g., Traveller, Star Frontiers, Thieves’ Guild, DragonQuest, The One Ring, Trail of Cthulhu, etc.). But the ones listed above are the ones that stand out for me.