If you're levelling by yourself, your best bet is the class & planet missions, bonus missions, the bonus series on planets which have them, and maybe some of the H2+s. You can also do some warzones if PvP is your thing, or some space missions. But there are more than enough missions for solo play to keep up with each planet.
If you're with a friend you can definitely do any of the H2+s (after They™ fix "Jungle Fight" on Belsavis; it's currently "not balanced for 2 players").
If you're in a full group you can skip nearly every side-mission (since you'll be doing 4x class missions), then roll through the bonus series and heroics.
However you go, just make sure you're at least level 33 before you get to Quesh.