I've personally never been much of a fan of cosmetic UI addons, but I'm a big proponent of utility addons and feel like they can improve your performance dramatically. Therefore, I won't really be talking about unit frames or action bar replacements because frankly I'm not very current with them. If you're interested in giving your whole UI a facelift, you may want to look into an all-in-one solution such as ElvUI, or check out WoW Insider's addon spotlight. Let's start with the addons I can't imagine playing my hunter without.
- Skada Damage Meter (Curse | WoWInterface): This one is my personal preference over Recount. I like being able to save individual parses for later viewing, and I like some of the plugins you can use with it. One of the best ones is SkadaExplosive. This one is entirely made for tracking the T16 4-piece survival bonus. It lets you know what your longest Explosive Shot streak was (every time I get a 10 streak I feel like it's Christmas morning), the highest damaging tick, and other Lock and Load stats.