Mythras (formerly RuneQuest 6 and Mongoose's RuneQuest II).
This is the game that I've played (as opposed 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.