Of the myriad systems I own, these have ended up being most dear to me (in descending order):
D&D (B/X, 2E & 5E, 3E for cannibalizing; I've played it for too long not to have as first)
Savage Worlds (if the game world doesn't have a bespoke system, I'm going to use this for it)
Vampire, and to a lesser degree Werewolf (Good memories from the 90's, and system that really favors interaction over combat)
Star Wars WEG (good, simple system that feels very Star Wars, and all the contributions WEG had to shaping Star Wars)
Star Trek Adventures (best Star Trek system I've found, especially solo)
Alien (It just does the universe right)
Blades in the Dark (took me aback with the system for "pre-planning that of course we already did" and event clocks)
Serenity (the CORTEX system is pretty easy and I like the universe)
Marvel FASERIP (best superhero system I've used IMHO)
Renegade's Transformers RPG (While character creation is hell, I wish D&D used its rule system)
Fading Suns (pre d20 era; love the Dune-like world, though the rules are bit flat)
Star Frontiers (my first introduction to a Sci-Fi RPG and simple at its core)
Gamma World (2nd Ed - will never forget my Rabbit PC with his landspeeder & mounted laser gun)
Twilight 2000 (the new Free League version; another good solo game and I just love poring over the military hardware of the original system)
Villains & Vigilantes (First superhero RPG I played in; while powers are quite random, it can cover any sort of hero and plays very D&D like. I was very fond of it until the Marvel game took its place)