Very few raiders keep the default UI, and I'm not one of them. It does a good job at providing new players with most of the information they'll need for leveling and even instances, but tends to fall apart once the player reaches endgame. It doesn't make the information you need readily available, while it displays a lot of extraneous information in ways that obscure the visual field. There are a lot of mods out there, though, and simply going to curse and trying to build a UI from scratch without any familiarity with the possibilities is an intimiating prospect. So I'm going to post a picture of my UI in a raid so you can see what decisions I've made and why, and what mods I used to get the results I wanted.
First, here's a sized-down version of the screenshot:And here's a list of the mods in the screenshot:
1. SLDT stands for "Simple Light Data Text", and is the most compact way I've found to easily and intuitively track a couple things. It's highly configurable and has many more modules available than I actually use, like bag space and FPS monitors. It displays more information upon mousing over one of its elements, such as the "Guild: 33" text. When you mouse over that text, a simple, scrollable list of everyone online in your guild appears, along with their rank, location, etc. You can click on a name to whisper that person or alt-click to invite them to a group. Mousing over durability displays a dropdown of the percentage durability left on each item slot. So easy!
2. Tipsy is a tiny, intuitive mod to move your tooltip around your screen. With all the other moving around of things that the other mods did, I needed to move my tooltips to keep them from obscuring other things.
3. Satrina Buff Frames, or SBF, is an easy way to configure display of buffs and debuffs. As you can see, I've configured my buffs to be smaller and and half-transparent, because that's good enough to check them before the pull, but keeps them from getting too distracting as they multiply and overwrite each other during an attempt. Debuffs, on the other hand, are big and bright, so the second I get something like Mark of the Faceless I know to disengage away, get out of the fire, etc.
4. SexyMap lets you resize your map, move it around, control which buttons display on it and when, anchor it or hide it, or make it sexier with cool glowing blue borders (not pictured).
5. Scrolling Combat Text (SCT) in combination with SCTD lets you define which sorts of events (heals received, heals cast, debuffs or buffs gained, reactive abilities activated) go to which of three "frames". I have Frame 1 display most of the stuff that happens (heals received, energy gains, damage taken, buffs gained) displayed to the left, in an upward scrolling curved hud configuration. I also use reduced opacity to keep it from being too much in the way - I mostly glance over at it to see if I'm, for example, getting mana gains from Judgement of Wisdom. Damage out goes to the right, and debuff gains scroll a message right under my feet in a larger font and at full opacity (again, great for things like Mark of the Faceless).
6. Pitbull4 is an example of what are termed "raid frames". Once more, I've elected to disable most of the mod's functionality. I choose to display my healthbar, my pet's healthbar, my target, and my target-of-target. I also have the focus frame enabled, and it appears above Grid to the right side of my screen when I have a focus target set.
7. Watcher is an ability timer. As abilities come off of cooldown they scroll from the right to the left, then stack up in order of highest priority first. When I took the screenshot, both aimed and arcane shot were off of cooldown. Aimed is displayed in front of arcane because it's a higher priority. This mod is great for two reasons. The most obvious is that it saves my eyes from staring at the teensy-tiny buttons on my action bars. The second, less obvious one, relates to steady shot's cast time. Steady shot does less damage than anything else I could do and, under raid buffs, it frequently has a cast time about the length of the global cooldown. So if casting a steady shot will delay a much more valuable arcane or chimera shot, I'll just wait the half a second or second and fire the other shot.
8. Quartz is, mostly, a castbar and swing timer mod. Sadly, I didn't choose the time of my screenshot ideally, or you'd be able to see the very large, obvious castbar displayed when my target is casting something. Very, very many bosses in the game cast or channel something as a warning that they're about to use one of their abilities requiring a specific reaction (Brundir's overload, Mimiron's laser barrage, Yogg's lunatic gaze), so I've made it very easy to see and respond to those abilities. I have a second, smaller castbar above that one for myself. Attached my castbar's location are my debuffs on the boss. This is more important for Survival hunters than it is for Marksman hunters, but it's still good to have an easy place to check to make sure serpent sting is on the boss. You don't see hunter's mark there because the other hunter in the raid has improved hunter's mark, so we both do more damage if I don't overwrite his.
9. Chatter just lets me resize and reconfigure my chatbox. Nothing too exciting!
10. Bartender 4 is a highly configurable actionbar/menu bar mod. It lets you resize, move, and change the configuration of your action bars, as well as hide and display them. I have my pet bar hidden, my mini-menu hidden but available on mouseover in the lower left corner of my screen, and my main action bars scaled down so they don't take up quite so much room. I also use Button Facade to reskin my buttons to make them a little prettier. This mod is especially nice because it works with Satrina Buff Frames too!
11. DoTimer, as the name implies, is mostly designed to watch your dots. It also, however, can watch your cooldowns, which is what I use it for. You can move it around, resize the bars, change the style of the bars, and configure the abilities that appear in a couple different ways. You can either have it display everything and then selectively block abilities, or you can set it up like I have, with a "whitelist." That means I list specifically the abilities that I want to know about. This mod helps me make sure I get the maximum use out of my cooldowns.
12. Omen threatmeter is an essential for any and every raiding tank or DPSer. Use omen or pull off the tank, die, and wipe the raid.
13. Grid is another, different raid frames addon. More popular with healers than with DPS, I still find it invaluable. From a convenience standpoint, it's excellent for summoning the raid to the instance, and you can also keep an eye on it to get an idea of the raid's average health. Further, as hunters, we can use it to do things like easily Misdirect to the tanks mid-battle or use master's call on a healer rooted in fire.
As you've probably gotten a sense of from my descriptions, my primary goal with my UI is for it to display the information I need in an accessible way without cluttering my screen or making it hard to see when I'm standing in fire. That's why I've made several elements semi-transparent and shifted a lot of the remaining elements to the borders of the screen, leaving the middle as wide-open as possible. Your goals and priorities for your UI may be different, but hopefully this post has given you some starting points so you can build something that works for you.