Quickest way to level 60-80?

[Ualaa]

First off, having the heirloom shoulders (Wintergrasp Shards or Badges) and heirloom chest (Badges) each increase your experience by 10% and stack.
So if at all possible (already have a team at 80th), purchase both of these items, for each toon on your team.
I have Cloth with Caster Stats and Leather with Melee DPS Stats, figuring that will cover all teams; even plate toons in leather is decent and worthwhile for the 20% increased exp.

From 60th to 70th, you could easily boost if you had a half geared 80th.
Probably 55-60 Ramparts, 60-62 Blood Furnace, 62-64 Underbog, 64-68 Sethekk Halls, 68+ Northrend... or something similar.

Questing from 60th to 70th is quite fast, if you have a flying mount, which you can buy at 60th now.
Similarly, 70th to 80th is fast with flying, if you have 80's to buy (Account Bound) Cold Weather Flying books.

[zenga]

From 60th to 70th, you could easily boost if you had a half geared 80th.
.

Thing is, what do you do with the toon on the tank account ..(as he says he has 5 at 60)? I have the same now with my new teams. Questing is sometimes very boring and i decide to boost my dk's/priest team with my very well geared pally tank. It's very fast with rested xp. However there is 1 toon behind. I end up running him through instances with multiple shammies, which (due to multiple lvls 80s) makes it way slower.

So for me the best way to level up is the way that you won't get bored. A mix between questing, battle grounds, instances and farming trash packs.