In general, the best boosters are (in no particular order): Druids, Paladins and Death Knights.
They're tanks and have good AoE attacks.

At lower levels, something like a Hunter or Elemental Shaman might be superior.
When you approach 80th on the characters being boosted, your booster needs to efficiently deal with bosses that hit a bit harder.

I've boosted with a lot of classes, and am currently using a Frost Death Knight for this set of RAF.



With RAF, it's very quick to do dungeons with the LFD tool.
That assumes you have a composition which can complete the dungeons with ease.

To use the tool you will need one toon who can spec into a tanking role.
A different toon who could spec into a healer role.
That's not saying you need to spec Tank and Heals, but if your composition could not, then the LFD tool won't let you queue.



A > B > C
A > D > E

Only levels gained on C can be granted to B.
Only levels gained on E can be granted to D.
Both B/D will grant levels to account A.

When B receives levels from C, half of those levels can be granted to A... ie., they cascade.
Same for levels gained on D, based entirely off of being granted to D from E.

If you boost four toons on BCDE, with A.
And then BD create two toons, and level them to 2nd level.
You can give two full toons, from CE to BD (that is 2+39+39=80 and 2+39+39=80, on both accounts).
Similarly you create four toons on A, and level them to 2nd.
B grants levels, getting each to 41st; then D grants levels to each of the level 41s, which gets them to 80th.

After C & E have granted their levels to B & D...
B/D receive two full characters each.
The two characters granted to B, can grant a character to A.
Same for the two granted to D, which add up to a character on A also.

If you boost four toons on BCDE, with A.
You end up with:
A = Booster + Six Toons from Granted Levels.
B/D = Four Boosted Toons + Two Toons each from Granted Levels.
C/E = Four Boosted Toons.