I suppose it depends on what you use Blender for 
generally, though, PowerSave and SimpleTabs are both indispensable for me personally. I also find the HDRI Sun Aligner super useful, as well as Batch Renamer. For animation and rigging, RBF Nodes is life changing, and there’s a onion-skinning add-on called Animation Extras, I think? Lastly, still for animation, the Dynamic Child Of Constraint is very useful
Thought of more for shader editor- Node Minimap and Node Tabber. For texture painting- Brush Toggle Add/Subtract