Depends on your tolerance for repetitive behaviour and how fast you want to get it done. As others have mention PotD and HoH, along with doing your dailies is probably the fastest way (for dps), but not necessarily good for your sanity. As a tank or healer you can more easily get into dungeons your own lvl and the exp is as good there as PotD and HoH. Squad dungeons are also pretty good for experience for lvls below 50, insta queues for dps, and remove the random factor of getting other players in dungeons who can be an annoyance when you just want to get through for the exp.

If you aren't looking to lvl super fast though, you could just pick a job to do various challenge log stuff on once a week (along with roulettes) while playing your main the rest of the time.