I've only started going to the dwarven lands, but my favorites so far (played mage and paly on hard to both areas) -
Royal Thorns - if only they could be resurrected ... stupid plant
Polar Bears - gotta love them as tanks
Dwarf cannons - love their initiative and salvo
Archmage - versatile unit... shield is awesome for melee units
Royal Snakes - love no retal
Marauder - $$$ , Rina + chieftan belt make them quite viable for damage and income
Sea Dogs - with Rina + chieftan belt these make fantastic units during freedom islands.
Demon (level 4).. - I got a few in Darion and with good use of sacrifice I can see the hype for all demon units
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Haven't got hold of level 5 units, beholders or shaman yet, but I'm sure they wont disappoint.