I currently use callcentric
and have been impressed with both, although I prefer callcentric.
I'm also testing callwithus
as they seem more flexible and have some cheaper costs - I can't really comment on the quality though.
Depending on where your calls are origination/terminating it's probably going to be better to have your server and your providers servers locally. If that's not possible try and go with someone that will allow you to 're-invite' the media stream meaning that the audio won't have to 'bounce' via your voip server once the call has been established.