I just searched for past threads on this topic, and seems time for a refresh. In the past I've had great solo bar meals at (no surprise) Craigie, ICOB, Coppa, Oleana, Strip Ts, Hungry Mother, and Highland Kitchen. I like the combination of friendly bartenders, small plate specialties, and (mostly) buzzing atmospheres at those places.
I'm looking to expand my horizons and / or check out some newer places. I work up in Burlington and live south of Boston, so basically anywhere in Boston, Cambridge or inside 128 works for me.