Everyone enjoys different things.

Only using 3 accounts, I've enjoyed playing 3 retris as an all melle team for questing since they have some decent self healing etc, you dont have that much downtime, you do have a proc of instant exorcism that you would want to use for oprimal dps though, check Elitistjerks for more info.

A Paladin tank is a good tank for multiboxing I think, you can have a simplified cast sequence rotation or tanking, which wont be optimal, but will work.

The basic rotation atm is 939, so just hitting 2 buttons alternativly to tank will do a decent job.

Button 1: Crusader strike
Button 2: /castsequence reset=combat Judgement, Avenger's shield, Shield of Righteousness

Don't macro it myself, but something like that should work pretty decent, won't work properly because sometimes Avenger's shield wont be off cooldown, that's when you should be using holy wrath although it does proc pretty often that you get an extra cast from the Grand Crusader talent. You can find out more from Maintankadin.

Pretty sure warriors and Druids are more based on procs than palas or deathknights. So if you are looking at doing boxing 5 man isntances you should look into getting a DK or Paladin tank.

Personally only done Prot pala + Resto shammy combo up to level 80 heroics with 3 friends doing dps.
And holy priest Blood DK up to level 75 with 3 dps friends.