PCSX2 ( http://www.pcsx2.net/ ) will likely be the best but I don't think it hasn't been ported to the Mac yet. Someone recently ported PCSX1 though so maybe one day. I don't know why some developers automatically miss Mac out. Mac has a bigger market than Linux I'm sure. Anyway, it's on Windows.

I don't know about playing copied discs. I read there was a way to copy discs so that you wouldn't have to mod your PS2. I just use originals anyway. When I owned a chipped PS1, I used to rent games and copy them so I got the games for about £3. Nowadays with PS2 games on DVD it just isn't worth it to copy them since I can buy them second hand pretty cheap and I don't have to mess about buying DVDs. Concerning backups, I have bought games that have been used loads of times from the second hand store and I've never had a bad game. Oddly, PS2 games seem to be more resilient than DVD movies. I've had tons of bad DVD movies from the rental store. Maybe I was just luckier with the games.

As for emulators not being good, well that's true in a lot of cases but the ones that aren't good tend to die pretty quick. Older console emulators like SNES, gameboy even N64 work very well.