Well there is only a few "must have" addons but there are quite a few useful ones.
MSBT: A scrolling combat text, basically you can put your damage done, damage incoming, heals and notifications to scroll down where ever you want on the screen.
Grid: Very light, customizable raid frames, pretty close to a must have.
Bartender: A light weight bar mod that makes it so you can move the blizzard bars where ever you want on the screen.
Omen: The only must have addon, it's a very customizable threat meter that can show with text when you are losing threat on a target, when it changes targets and when it is attacking you.
Forte: A nice cooldown watcher.
Acheron: A death watcher, you can click on it and shows you everything that happened to you up to the last 10 seconds of your life.
Pitbull: More cosmetic, just generally look better and are more customizable than the standard blizzard ones.
CBH Viewport: It can make the top or bottom of your screen black so that the edges of your screen aren't consumed by addons.
Satrina's Buff Frames: Also cosmetic, very nice buff frames.
Eerror: Removes all of the annoying " You don't have enough mana" red text that blocks off quite a bit of your screen.