I do class quests, the occasional side quest/heroic, and KDY. Class quests are entertaining, different per planet and class of course, and it's the only way to get your companions other than HK and Treek, so if you want to craft, you need to do them anyway.
Grab the Legacy XP boosts for class quests and flashpoints for KDY, and you almost won't have to do any other side quests ever. If you find yourself getting a little behind for the planet your on, grab side quests/heroics and use an XP boost you get for free when turning them all in.
Spending 40-50k on gifts for your companions to get their affection to 8k also unlocks their missions, and with an xp boost you can get upwards of half a level going through all of a comps missions at one time.