As a mostly solo Gurps player, I had the most fun running a modern TL/8 campaign with some elements from TL/9. Having Flex Armor especially makes modern fire-arms much easier to deal with.

My runner-up would probably be TL/4 with some magic elements here and there. I don’t actually own Gurps Magic or Thaumatology, but creating/designing magic items is fun regardless of what system I run it on.