I've seen both excellent and crap divers from pretty much everywhere, and no one continent stands out. I've seen great British divers, but I've also had to watch a BSAC instructor bounce off the reef endlessly and then explain to him why his air-only computer locked out after he'd blithely followed me on a nitrox profile. I've seen awesome CMAS divers, but had to tell one Belgian CMAS 3* diver that, no, his NDL didn't get longer as he got lower on gas: that increasing number was actually his decompression obligation, and his computer was beeping at the surface because he hadn't cleared it. I've dived with Russians whose idea of gas management was 'suck tank dry in 15 minutes, swim to dive guide at 25m and signal out of air, expect him to get me to surface.' Hilarious, until the third member of a group of eight does it and you know the other five - who have all been ignoring your 'requests' to ascend the line since the first one went OOA - aren't going to be far behind. Again, though, I've dived with superb Russian divers, too. Americans I've dived with have also run the gamut from utterly horrifying to extremely competent.
The one real difference I've noticed between Americans and Europeans/Australians diving here is the response if I say I think a dive is beyond their experience level. American divers seem much more likely to insist, based on the experience they've gained in a handful of dives, that they'll be ok with the dive I don't think it's safe for them to do. Although I'm talking about a very small sample, so I'm not sure there's any great insight to be had there.