These days I don't really run a system per se. I like a lightweight frame that I can patch other bits on to as I need them.
Whitehack, Dungeon Crawl Classics, Lamentations of the Flame Princess, and especially Pariah are all ones that had a significant impact on how I run games now. There are also several games outside the OSR space that have likewise influenced me heavily.
Though if I am evangelizing it will often be off the back of Cairn because then everyone can have a copy.
Warlock is a rules-light version of warhammer fantasy roleplay. The core dice mechanic is d20+skill. Combat rolls are opposed rolls, sword vs mace skills for example. It uses the career system from wfrp so characters start as something like rat catcher and change careers to advance. If you like the warhammer old world but don't want a crunchy game, I'd recommend it.
I grew up playing B/X and then AD&D. Right now, I'm running Swords & Wizardry with a couple of house rules and it's really feeling like my forever home.