My friends and I started off playing D&D 3.5e in high school and college. We played it a lot over the years. Eventually we started playing 5e, and that's mainly what we play today (on the rare occasions we actually sit down and play).
I had a friend who enjoyed experimenting with other systems like "GURPS" and "Swords & Wizardry". We only ever played a few sessions with "Swords & Wizardry" but I enjoyed it a lot. It was an extremely lethal system where players were encouraged to be clever rather than solving problems through comat, since players weren't particularly powerful compared to your typical monster or NPC.
Most recently, another friend introduced us to "Apocalypse World", which we really enjoyed. We originally were just gonna play a one-shot, but wound up having a dozen or so sessions to keep the story going.