Which faction poses the greatest challenge for the Dwarves?

I agree with JackFromShadowLand, luck is one of the most important skills for Dwarves and should be taken as early as possible. It compensates for the Dwarves' low damage (at the start)... and later, especially with soldier luck, it activates abilities: bears + stunning claw, throwers + crippling, archers + stunning strike, larger units + increased damage, priests + sign of fire... plus, of course, runes :)