Feral druid is good fun, Sub rogue is fun too (haven’t tried outlaw/sin), BM feels like rubbish (been happily playing BM since WotLK) so I switched to MM which is actually good fun. Arcane mage is alright, it becomes more fun during clearcasting and when you can spam Arcane Barrage for speed boosts. Prot war is only fun if you can use Dragon Rush. DH is alright.
I like to move around a lot, the faster the better, so feral druid and hunter w/ engineering are always at the top of my list.
Blood DK felt like absolute rubbish. I mained DK during WotLK and Cata, but at 110 it feels so horrible. Hopefully they feel better to play at 120, but I’ll likely never play one again.