Would love to recommend Wuss, but the stipulation of being relatively successful in terms of trophies is a little off the mark for us, lol.
I'd probably say Saints for the same reasons said before- Great team (Chris Ashton is going to be huuuuuuge at the World Cup), great fans, fantastic stadium, they've won the Heineken Cup in the last decade and reached the final this year, and possibly about to challenge strongly for domestic honours (if Leicester and Sarries drop the ball a bit).
Also Northampton is a nice city; it's not exactly a thriving metropolis, but it is defintiely a nice place