Druid/Pally is pretty sweet, especially lvl 1-30.
Outdoors the druid can kite while the pally adds dps.
Indoors is great, because the paladin can bring in single pulls, and can also help heal to lower down time.
Having evac available in a dungeon while duoing is a must, you will feel much safer to go further inside
and paladins having LoH can really make the difference saving yourselves in a bad pull/train.
Then come end game, you can port anywhere, and rez too!
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P.S: The problem with warriors, is unless you are twinked out,
you won't hold very good aggro, and your caster will get pummeled alot often when they nuke a mob or heal you.
However the exp penalty does suck, and may detract anyone wanting to duo early on.