Are you talking about botting or in general?
The fastest way I know of to lvl is to use the refer a friend bonus with a full heirloom set and quest while partied with the other account, while constantly staying queued for dungeons.
The RAF bonus gives you triple xp, the heirlooms give you another 30% (I think).
I've hit 90 in less than 6 hours with this.
If we're talking botting - Dungeons are the fastest way, but a sure-fire way to get you banned. All it takes is one time where the party does something abnormal and your bot starts acting crazy. You get reported and ban-hammered. Not only that but the ban will probably come weeks later when Bliz finally gets around to taking action, so you have more to lose.
Recommended method is kicks profile. Only bot for a reasonable amount of time. 16-20 hours a day are going to get you flagged.
To be fair I only bot because I've leveled so many chars at this point that I'm sick of it. I just want to play the end game content. Many people bot to make gold (or $$$), so they may all have different advice.