With more Patches released, the maximum level upgraded from level 60 to level 100 now, and it will up to level 110 in the next Legion. Then how to reach a level 100 in the shortest time? Buy a level 100 in Blizzard Store directly, or level it by yourself to feel the game fun slowly? For a new player, the best way to upgrade is to do quests and dungeons, which can let players familiar with the game and game operations.
Then for the experienced wow players, how do they upgrade fast? Upgrade leveling speed or kill monsters for experience? Heirloom, XP Potions maybe the first choice. 
The most one is Heirloom. Added lvl1-100 gears and lvl91 BOE weapon, you will grind the wild monsters. 
Excess Potion of Accelerated Learning - Experience gained from killing monsters and completing quests increased by 20%.  bought by Garrison Resources when to reach level 91.
Elixir of the Rapid Mind - Increases experience gain by 300% for 15 min. Does not work for players above level 99. It's getting from Garrison Quest, also can be bought from AH by WOW gold.
Elixir of Ancient Knowledge -  Increases experience gain by 300% for 1 hour. Does not work for players above level 84. It is a rare drop.
With using the potions and three times XPs from friend recruit, you will get dozen times XP from slaying monsters.
Now with a dozen times XP, isn't easy to upgrade levels for an experienced wow player?