Scythe or Spiked Chain take the top spot.. Sure, either way you go you're giving up about 4 damage on a maxed out damage roll...
Big freakin whoop, they've got stuff to MORE than make up for it.
Spiked chain.. You can attack from 10' away, and when they move forward 90% of the time you'll get an attack of opportunity, then take YOUR full attack.. and back up 5', to repeat..
SO, you're giving up an avg of 2.5 damage / attack, but get an extra potential attack each round.
PLUS tripping+disarming, and when you get into Improved Whirlwind/Greater cleave at epic level? OH look, an army of hundreds... *6 seconds later* Oh look, dead bodies.
As for the Scythe.. just amp out your crit range, head into weapon master, and make your opponents fear high rolls on the dice. You'll more than make up for the measly few points you're missing each turn.