I've played most of the popular class combinations to around lvl 60ish.
The most fun I had/having is with with Champion/rogue

. Unlike some classes a champion can be viable as dps or tank. There are a few different combos you can perform these tasks through class combinations with the champion, but imo C/r lets you do them the best. Some classes just spam a few buttons, champion gets to use their skills based on how the fight evolves. This makes playing, learning, and killing with the combo all the more fun
After 1 and a half years of playing this game champion/rogue is probably what I'm gonna finally try to take to the next level
C/R > all the others (well maybe not druid or mage :3)