For leveling Questie is pretty useful unless you want that classic feeling of having to read quests and look things up to find where stuff is. Pawn is also nice to get the vendor price of items in your bag without being at a vendor.
Auctionator is basically essential in my opinion if you want to buy things from the auctions house without having to do a ton of searching for what is actually the lowest priced item.
I know for raiding some people will use a lot of add-ons but at least personally I only use Details for damage meter, DBM for raid mechanics, and Weak Aura for some class related mechanics and raid mechanics.