My leveling thoughts.

20-90
use Azeroth Auto-Pilot (AAP) addon. This is just a horrible grind even full herilooms and exp pots, so boring.

For rest of the leveling, don't use AAP. It assumes you're solo and skips all the tough stuff, which multiboxers dont worry about.

90-98 The funnest leveling range.
WoD bonus areas. Get yourself a map from Google Images and hit them all. You will be level 98 and ready for Legion before you are even done with all the bonus areas. Kill any rares for exp you find on your way, but its not worth going out of your way to kill a rare.

(98)100-110 Another really fun leveling range
Legion bonus areas and invasions. You will gain 2 to 3 levels for every invasion you do. You will need to do about 2 invasions in addition to the bonus areas to hit 110. The rares not worth doing, take too long to kill but good exp.

110-120 Yet another fun range
I only log in for faction assaults for this range, after an assault you get another exp buff that lasts an hour. Using the exp buff to do the bonus areas and the next faction assault really speeds things up.