Caveat: I've only ever played an elf shield paladin with two virtues, so my perspective may be skewed.

Shield: excellent melee defense, can dodge some breath attacks, can stack on lots of built-in damage reduction, can cure disease/poison/blindness, and can heal. Also can sanctify room, which is big since so many areas are cursed.

If you're maran, wield a wrathing weapon and use phoenix brand to make any mob vuln_holy. Then you can spam angel's wing if you want to conserve mana, or spam wrath to kill it as quickly as possible.

2H: paladin has worse melee, but can do big damage moves for "free" in terms of mana.

I haven't played monk or the polearm one so I can't comment on how they stack up vs. the other two w.r.t. PvE.

I will say that the "marshall" skill on the polearm/staff type is pretty sick if you're with a group that has a lot of melee type guys. You'll see warriors getting 6-8 melee hits per round. But that doesn't help if you're solo.