Blender addons

Loop tools, Node wrangler, UV packer are free and good.
It depends on what you are specializing in. It does not make sense to download addons for animation if you only do 3D modeling.

Hello there. It depends on what kind of tasks you're up to.
You can do ANYTHING in blender without addons. Just pick a direction of what you like the best - then take add-ons and plugins to speed-up your work.
Hard ops - for hard-surface modeling
Botanique - for faster trees and plants scene population
There are more (of course) but it would be easier to give an advise if you specify the modeling task.

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