Tip tier rhodok crossbowmen, as in native, are clearly the best ranged siege unit IMO.
Their advantages over archers are numerous. They've got shields, making them more survivable while positioning as well as better melee units(in fact they're better melee than many T5 and even 6 melee units I'd say). They have higher armor, making them more survivable in general. I'm pretty sure they deal more damage per projectile than archers, which in a long siege is superior to faster firing rate if running out of arrows/bolts is an inevitability.
As for the lower tiers, archers will kill weak enemies much faster, and archers can kill crossbowmen faster than the other way around. Still, low level crossbowmen have piercing damage while most archers won't, making them the superior unit vs. higher tier enemies.