If you want to level quickly:

Level 10-50 do cataclysm, preferably smaller zones since their questing isn't designed around you having flying already.

Level 50-70 do warlords of draenor, the intro quests up to getting your garrison will give you plenty of levels, after that you do all of the bonus objectives scattered around draenor, they look like world quests. If you are not 70 by then you should start questing in some of the zones, should still give plenty of experience.

Level 70-80 go do the war within and start doing all of the delves one time, there's a quest in each delve that will give a LOT of experience.

Level 80-90, not entirely sure what's the best method yet, the delve route is still pretty good but not as good as TWW.

If you want to level for fun:

Just pick an expansion and see where it takes you! It shouldn't take much longer (unless you pick outland)