Well, there are several sides about addons, e.g.:
And the addon section at Blender Market will give a good overview as well.
About my must have list.
Shipped with Blender:
Nodewrangler, Looptools, ExtraObjects.
When I’ll start to learn rigging/posing in Blender, Rigify might become useful as well.
Other must have addons:
GoB and DAZ Importer
I also bought some addons lately, all of them are nice to have, none of them are essential:
Hops, Fluent, Transportation, Simply Cloth.
But of course this depends on what one is doing with Blender
and what other software and tools he use.