depends on the type of bow undead or demon slayers 4 sure keep if the bows are the slower types like heavy 30+ ssi if it has another good mod i would keep like 30+ mana leech also stamina leech is popular any with lightning or fireball of 30+ plus ssi or hld good basicly your top mods are
slayer
SSi
mana leech
stamina leech
hld
hit spell
hci (think the max on this one is 17 not sure though)
if you have a bow that has 3 of these mods at 30+ keep them
the least desired mods usualy are hit area, resist mods,mage weapon (although some people do like this mod but not many) etc
hope this helps