Stormheim is the zone you want to start it. For some reason Legion treasures are not worth any experience like they are in WoD...except in Stormheim. I haven't been able to find out any reason for this, and it's possible it may get fixed/nerfed at a later date. But for the time being you REALLY want to jump from treasure to treasure to augment your quest experience.
Pick druid, paladin or monk. Get heirlooms if you can. Grind mobs until level 5 - 6. Start doing the easy "just kill these mobs outside town" quests until 15. Queue rdf as every role and do every dungeon you can (with heirlooms you could probably solo the dungeons) while you travel to higher lever zones. Keep doing the simplest quests while you're waiting for rdf (which shouldn't be long). I leveled a druid to 100 once doing this in maybe a day /played, but he was also rested for most of the process.
These two expansions are virtually identical in terms of exp/h. I personally favor Mount Hyjal, and think it's very slightly faster. But if you know Jade Forest well you can just as easily keep up there. Ultimately you'll end up doing both anyway, so it's mostly academic. Every time I've tested Valley of Four Winds or Deepholme, or any other zone in MOP/CATA's level range, they were worse exp/h. You should not even need to complete both zones to ding 91, so it's more important to just pick one and go.
I guess maybe I'm familiar enough with it to just slam it out. I don't recall any low droprate quests as bad as 5%. Although Blizzard has been screwing with stuff lately. Do you remember which quest it was? A lot of those can be skipped if they're too tedious. Like literally ALL the quests on that little island in the NE corner of the map with the ghost pirates underwater and the dragon whelps.
It still will be but it takes up an Azerite armor slot. ST is not as important as AoE when leveling because you want to grab a pack of mobs, burst them down, loot and repeat. For DK specifically, Unholy is better in my opinion for leveling because it has a lot more aoe than Frost with epidemic. Unholy was my second fastest leveling time and I did it on a pre-made character with no leggos or consumables.
As you're leveling, I generally recommend you just follow the natural flow of the progression through each zone. They're laid out fairly reasonably. But don't be afraid to hop around, especially if you're near a quest objective. You shouldn't actually need ALL of the zones listed to reach 62. Pick the ones you like best. It's much more important to simply be as focused as possible and flow from one quest to the next.
Usually it's best to avoid a lot of instance runs. It is too difficult to account for bad groups, and some instances are too far away with too few quests. Since the experience is very good, it's a good idea to clear nearby instances which have a lot of quests once (Gnomeregan, Deadmines for Alliance). Distant instances with few quests should be avoided (Deadmines for Horde). Low level instances can be cleared by paying or befriending a single high level character. However, since patch 3.3 a new Dungeon group finding system has been implemented, as well as a direct port to dungeons (when entering the group you get ported to the dungeon and back to the same location if you leave it) and thus the travel time to dungeons has been greatly reduced. This makes it a good idea to complete quests or grind while you wait for a group to become available.
I just decided that I would simply finish leveling my hunter the hard way, but when i can get heirlooms ill make a warrior and follow your guide to 80. I definitly want an 80 warrior for cataclysm... seeing as worgen warrior is going to be a crazy tank! By the way, is using heirlooms with your guides ok? And what class should I be for a tank? I will obviously be switching to worgen anyway but i would prefer best tanking until then. Thanks Jame!
I made one for my guildies but its for the Horde leveling process. One tip that goes for both factions is that you dont need to do all 3 zones to hit 120. I did like 1 and than three quarters of another. But once your 120 you need friendly with all 3 of your factions new reps to unlock world quests so make sure you hit at least friendly with all 3 while your leveling.