Very hard to say.
I've done OK with 32Red and Ladbrokes (RIP), while other affs say these have been rubbish for them because players see them on TV a lot and sign-up direct.
To be honest, I don't actually check deeply enough into my stats to see where my players are coming from (because I work with about 70 programs and check all my stats manually!). I don't geo-target either.