Well there are a lot of variables, what is best for you does not have to be best for me, do you want loads of damage from the bow, do you think added elemental is good? Do you play with a tenebrium bow, or did you take ranks in bow or crossbow and therefore having less benefit from a tenebrium weapon. Are you looking for particular status effects that can be applied...
"Best" is rather relative, but if you want to be absolutely safe, then my answer will be dex 15 and you can equip the strongest crossbow you can find, it will have the highest base damage a (cross)bow will have. Will it also be useful, no... but it is the best.
Rashar.