Probably…
1) HERO 4th
2) HERO 4th Ultimate Martial Artist
3) Space:1889
4) Paranoia
5) Traveler
The logic? With the first 2, I can run a simulation of nearly any setting I can remember or imagine. The last 3 have reasonably distinctive settings I’d love to be as faithful to as I could be If running them in HERO, but would also be good to run in their native system. If this list were expanded, I‘d add the core books for games like Deadlands, Underground, RIFTS, Shadowrun, etc., with the same motivation.
I also might change the list to a set of key HERO 5th books, with a shorter list fr9m the other games.