A tempered Bostonian. If it is formal, Broadcast English. If I'm out and about, I'll do a mild form of the regional accent, as appropriate.
Off topic, but the missus gets a kick out of it. It's a valuable skill. I do a mean Down East accent, which is different than the rest of Maine. (I live here now.) With practice, you get an ear for the variations.
One of the best exercises was learning to speak with different accents in different dialects. This has actually come in very handy.