From my own personal collection I have the following from D&D>Rules Cyclopedia>2e>3.5>4e>5e (most complete collection)
For other systems that are related to D&D>BFRPG>LotFP>DCC RPG>Darker Dungeons>For Gold & Glory>Strike! RPG>Dark Places & Demogorgons>Pathfinder
For games decoupled properly from D&D I have>WHFRP 2e>The One Ring RPG>FATE Accelerated>Battle Century G/Z>Kids on Bikes
I'm expanding my collection every month. Really, damn near any system is playable (notable exceptions being VTNL, FATAL, RaHoWa and a few other minor ones), even D&D. I find that D&D is more like 5 systems all tacked together (except 4e which meshed nearly everything together really well) and you have to pick which of those systems you want to focus on. The other systems I own do a really good job blending the various systems to various degrees.
Is there a specific type of game you're wanting to play? Then find the system that you think would work best for it. Like, you should use D&D for a Call of Cthulhu game just as much as you shouldn't use Star Trek Adventures for gritty noir detective game.