For me that would be:
Astute Buddy: Quick help for all the plugins
SubScribe: construction
ColliderScribe: Snapping
Phantasm; vector halftones and Photoshop-like color editing
VectorScribe: pretty advanced effects, Round corners effect and Smart Point Eraser (I'm not using the other tools it has a lot, but its pen tool replacement can be pretty efficient as well)
FindReplace
You might want to use Autosaviour for your backups
And if you need to edit a lot of other peoples' files, then definitely VectorFirstAid (see Bobby's reply)
Then there are some that I'm not using a lot, because they are aimed at very specific workflows or styles. But if those are the things you want to do, then those are the plugins you want to install:
Stylism: various special effects - if Blockshadow (long shadow) effects are your thing, you will need it
WidthScribe: all sorts of impressive things around variable widths, e.g. woodcut-like effects
MirrorMe: nice for Mandalas
Stipplism: stipple effects
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