Oof. Depends on what I'm in the mood to play actually. In no particular order,
1. Marvel Vs. Capcom 2/Hyper Dragonball Z/Def Jam for a quick fix
2. Fight Night round 3/Punch Out
3. Morrowind for when I just feel like strolling around causing mayhem or GTA2 for light, cartoon mayhem
4. Rune Factory: Oceans or RF3 or when I feel like doing something monotonous
5. Outlast/Hide/Imscared/7Days for some quick horror fix
6. Left 4 Dead 2/Alien hominid/Firemen (SNES) for a run-and-gun feel
7. Backyard Basketball/Super Monkey Ball Jr. Golf for a quick basketball fix
9. Mario Kart/Road Rash/Rollcage/SplitSecond for light racing
10. Space Cadet 3D
11. Summon Night on the GBA and DS or Shining Soul
12. Frets on Fire or Bust-a-Groove 2
13. Ivy the Kiwi for when I wanna just relax
14. Ninja Gaiden and Shining Soul on the DS (works really well on a smartphone)
15. Dark Messiah of might and magic
16. Newgrounds' Mastermind: World Conqueror and Newgrounds Rumble
17. Tavern Rumble
18. Red Eclipse
19. Culdcept
20. Call of Duty: World at War
21. Half-Life 1, 2,
22. Portal 1, 2
23. Stanley Parable
24. Robot Arena 2
25. Tony Hawk/Jet Set Radio
26.Commandos series
There's more, but I can't really recall the titles, especially the older ones. Like there's this one where you start off as the zombie that starts the outbreak. And also where you get to play as one of 3 knights that can control the undead. There was also this other game where you'll try to save a possessed girl but you're blind so the game is pure audio.
Sorry; I know you said of all time, but it's hard to pick one when all of them play so differently that it's a matter of an itch for me.