Oh wow, fav river would probably be the Dargle or Glens in Ireland, both scared the living piss out of me the first time and have just become more and more fun as I've paddled them (not often enough on the Glens, unfortunately), I'm kind of hoping that they're going to be the kind of run you never ever get bored of or start to think you've run all the fun lines on.
Abroad it's got to be the Egua, best river I have ever been on without a doubt. Perfect low volume run, tough enough that you are loving every single moment, but not full on crazy to the point that you're just getting through any given rapid and hoping that the next one isn't completely over your head and you're starting a long and ignominious walk out. Just brilliant!!! The Etive Sara would be a close second, just a super fun run - first time over the triple step was just the best start to a holiday ever...