The British male voice (Harry Hadden-Paton) is actually my favourite out of the four, with the British female (Alix Wilton Regan) coming a close second. The vast majority of the time (when given the choice in a video game) I play as a human, and their voices happen to suit my characters brilliantly.
My first playthrough was as a male, human warrior and HHP's delivery was spot-on for the vast majority of his lines, in my opinion. I particularly loved when he was being humorous/sarcastic - his delivery was just perfect at times (the first conversation you have with Sera at Haven springs to mind, where you can say how ending the Mage-Templar war should be easy, and was easily one of my favourite lines in the whole game).
My second playthrough so far has been as a female, human mage and I'd say that AWR's delivery is mostly spot-on as well. The only thing that takes her down a notch slightly in my book is when she's trying to be humorous/sarcastic - she just doesn't quite do those lines as well as HHP does them (the aforementioned line with Sera is a prime example. I mean, it's still funny, just not as funny when HHP does it, for me).
Maybe it's some subconscious bias as I'm British myself, but the American voices by comparison don't even come close to the British ones. When I hear HHP or AWR speak they sound quite charming, natural, refined, and sophisticated; when I hear Jon Curry or Sumalee Montano speak they just sound so bland, monotone, and it seems to me like they're trying too hard (I particularly dislike Curry's voice. I don't know what it is, but there's just something about his voice that makes me hate it for some reason).