It doesn’t even have to be an add-on that you use often.
And you can only list one ;p
What is that one add-on [external, not built in], that if it is no longer available, Blender will be dead to you ? ;p
My own submission: Free add-on: EdgeFlow.
Not built into Blender, have to be installed manually.
If this add-on is gone, it’s almost goodbye Blender for me…almost.
Close to it in second place is “forgotten_tools” where Control + D afford one the power of 3ds max’s connect tool that puts Blender’s Control + R to shame.
Houdini. 
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WonderMesh I guess… Ability to change amount of segments on cylinders even after you clicked away any time is just too powerful
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Yeah, 3DS Max does that by default.
None, I try not to be entirely dependent on addons. There are some I really like to have though. TinyCAD for an example.
Ok I saw TinyCad in Blender 3.0 preview…wondered what is that…I will have to find out about it soon.
Grease Pencil…Geometry Node…and now TinyCad…I just want to crawl back to my poly-editing cave,