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.