It depends if you want to boost damage or immobilize.
For boosting damage, it is tough to beat shock damage (with an obligatory second of soul trap to help recharge).
Immobilizing bows are great fun for a poison using mage. Here are a couple options (remember to add a second of soul trap to any bow):
1. Demoralize + Turn undead. Both effects at max magnitude and for 1 - 2 seconds. When shot, the foe stows his weapon, turns to flee and takes about a step away (all while you are loading the next arrow). Then the effect wears off, they draw their weapon and start to charge - right into the next shot. In effect you can keep them dancing between the run away and charge animation. Caution: If the foe has friends around, they can still swarm you of course so it works best vs solo foes.
2. Drain speed 100 x 1 second. The real beauty of this effect is that when you drop a foe's speed to zero, it also brings his mind to zero - that is, he goes stupid for a second and 'forgets' that he was just shot. If you are hidden when you shoot, you will remain hidden after impact. This is a very very deadly bow for a sneakster. The fact that the bow actually stops a foe cold for a second is really just a bonus. Getting the effect in your spellbook (and thus, available at the enchanting altar) can a bit tricky. There are no spells for this available for purchase, but there are a few ways to get the effect. My character simply did the Orrery DLC quest to get it, but there are several other ways to get it. Note that if you know any 'drain attribute' effect, you can create spells/enchantments to drain any attribute. If interested, this link gives details: http://www.uesp.net/wiki/Oblivion:Drain_Attribute