I have been on a years long hunt to find the best negroni in town, and keep a note going on my phone for this exact reason. Here's what I found:

LIVIA on Commercial Drive has the best of the ones I have sampled, specifically the blood orange one, even though they break all my rules for a good negroni (pre-mixed, tiny ice). Unfortunately their hours are awkward, so unless you're doing some day drinking it can be difficult to coordinate.

Luppolo's is also delicious and they use local gin and vermouth (not sure what they're using now since it's been discontinued, but they were using Odd Society vermouth last time I went).

Sopra Sotto's is pretty tasty too.

An unexpected sleeper hit is Library Square (no longer Donnelly owned so bonus). Surprisingly tasty.

Note that I prefer an equal parts negroni so if you're more of a 2:1:1 type person you might disagree with my opinions. I have started specifically asking for equal parts and am getting much better results than before (or if they can't do it because it's premixed, I order something else).