As someone who has tested the ul dah and crab +3/2/1 the order is:

crab+3>crab+2>ul dah > verdant > crab+1

The hard math pretty much sums it up:
Crab bow +3
Att: 146 (127-base)
Acc: 190 (165-base)
Critical: 135 (117-base)

Crab bow +2
Att: 140 (127-base)
Acc: 182 (165-base)
Critical: 129 (117-base)

Ul'dahn Longbow
Att: 141
Acc: 166
Critical: 128

Verdant Bow
Att:128
Acc: 182
Crit: 121