GTA 3 series and Tony Crowther's Captive on Amiga or AtariST emulator.
And I occasionally go all the way back to a game of M.U.L.E. on C64 emulator.
Then there's ATC Simulator 2 (air traffic control), which has been haunting me for years, and UT 1999.
All these I ignore for a while playing newer stuff, but then I get one of them out again and go on an extended binge.