Honestly depends on the enchantment, but for now I'll say the Dawnguard crossbow. It's such an underrated reward for joining the underrated faction, and in my opinion sees far more use and is more practical than an overrated vampire lord form. Despite having used it for nearly a fortnight, the control still takes some getting used to and it can cost me valuable health and time. Holding down R1 usually draws the bow string back and releases it, but with the crossbow it points the weapon forward, even though it's readied and primed. Then reloading it is a pain in the backside because, once again, the control differs. It goes against every instinct you have for bows in any video game.
But the design is very nice and the upgrades are awesome. In fact I found myself doing those upgrade quests way more than the main Dawnguard quest line because I wanted my soldiers to have the best equipment. Having not yet completed the expansion I can't say if my soldiers use them or not (I would expect so), but having the Enhanced Dwarven Crossbow with something like an undead obliterating or fire damaging enchantment is the best thing when it comes to ranged firepower.
Overall a very good weapon no matter which version you get, and with elemental crossbow bolt weapons it's a doozy. The crossbow is my favoured bow-weapon, despite the uncomfortable and unnatural feeling control.