Depends on what you want. If you're just wanting to sit and drink a beer, I often get in and out of breweries pretty easily without any interactions outside of the bartender. Nightshift at the Garden particularly.

Tavern in the Square for a burger and watching a game, maybe talk some ball with the dudes next to you.

If you want a plate of spaghetti and a glass of wine, Trattoria Il Panino in the North End is my fave. They have a really long bar where it's easy to grab a spot and hang solo.

If you do want to get chatted up, go to the Faneuil Hall area bars. Bell in Hand, the Point, Green Dragon. Tourists make their way in there, especially BiH. They will for sure talk to you. They find a guy drinking alone mesmerizing and often think there's some mysterious story you have to tell or want to hear if you have the accent (both no in my case).