[PLEASE NOTE: This thread was originally started on the Boston board at http://www.chowhound.com/topics/show/..., and we've moved part of it to this board so the non-Boston lobster roll lovers can add their input-- The Chowhound Team]
Hands down the best lobster roll I've ever had in Mass. is at a tiny watering hole known as the Raw Bar (or just Bobby's) in the Popponesset Marketplace (Cape Cod). The lobster roll is a meal and a half. Little to no mayo, great chunks of lobster. The bucket of steamers and cobbed corn are worth it as well. The atmosphere is lots of fun if you hit it at the right times. I'm going to try Neptune as a new adventure, as others have suggested it.