AmiMasterGear is still my favourite. You can plug a real Sega Master System pad into the Amiga to use, and it runs perfectly on my 030 A1200. Installing this all those years ago opened up a whole extra world of slightly enjoyable console games for me.
On PC? I guess WinMAME is the only one I use to actually play a game, since there just isn't much choice since arcades all died out. I own most of the other systems I would play games on (Amiga, Mega Drive, SNES and Playstation mainly). The Mega Drive emulator for the PS2 isn't too bad though, I forget the name of it now.