WinUAE coz it utterly amazed me (and I love being able to play games on it that my unexpanded, HD equipped A1200 can't manage) & CCS64. That's both of my most loved compootas emulated. I downloaded ZSNES for the Missus and was thoroughly impressed with that too!
I had heard you can get the old Arcade classics emulated too like Robocop, Street Fighter, etc. but can anyone recommend any good ones and where to get the games from?