Definitely the Atari 5200 emulator of choice. It requires the Atari 5200 boot rom, available from Zophar's Domain. Or in the rom pack. This is the first Atari 7800 emulator, written by Dan Borris. The code for this emulator was originally written by Dan for MESS, but he decided to release a stand alone emulator while MESS is still in development.
Games are playable and it does have sound.