What are you doing in the game on a normal basis? PVE; dungeons and/or raids? PVP; battlegrounds and/or arena?
For PVE I'd say Deadly Boss Mods is pretty essential for boss ability timers etc.

i have a ton of addons, but most are only very minor quality of life improvements
the only ones i wouldn't do without are bartender, deadly boss mods, and grid probably
bartender lets you move all your buttons and bars around to where you want them
dbm just tracks pve timers and boss abilities
grid makes the raid frames more tidy and easy to manage

just browse curse, check out the most downloaded add-ons, and try some out

TradeSkillMaster is a bit harder to manage than Auctioneer etc, but it's well worth it imo

Deadly Boss Mod if you plan on PvE

TitanPanel is great imo

Recount as no matter what you are doing in the game it's always good to know how much dps/healing you are doing.

All you need is TradeSkillMaster for Auction House stuff

LUI ( Complete new interface because screw blizzards decade old interface claner and gives more info)
Bartender ( Because screw the shitty blizzard bars)
DBM ( for the sake of your raid team get it )
Garrison commander ( must have if u plan on using garrisons)
altaholic ( must have if u have tons of characters)
Healbot or grid ( wont play with out on a healer just makes life so much simpler)
Zygores guides ( makes lvling lol mode for someone new to the game) ( can also enable lazy mode features like auto hand quest quto accept auto equip an more)
Jamba ( for multiboxing makes things alot simpler)
TSM( all your AH needs in one addon)
Skada ( so u can see your dos/hps an other info)


Of course no Addons are must haves but they cab make the game alot better looking, enhance quality of life and help you progress faster.