I've recently completed Mega Man 2 finally for the first time ever. On my NES, with the original cartridge. Needless to say, great game, but I knew that already.
I'm also occasionally playing either Super Mario World or Super Mario Bros. 3 on my SNES recently. I have the All-Stars + Mario World cartridge so that's convenient.
I have been consistently playing a couple games on PC for a long time now though. I've been playing Rollercoaster Tycoon 2, SimCity 4 and Anno 1602 ever since they were released over a decade ago. I'd consider those 6th and 5th gen games respectively.