What is the best GPS for Boston?

"The Big Dig is problematic for nearly all GPS systems so..". You're obviously referring to the underground portions, which naturally will pose a problem, simply because the satellite signals won't the ground. No GPS can help you underground, no matter what is claimed. Above ground, just about any GPS should suffice. If pinpointing altitude etc. is important to you, try like a Magellan Explorist handheld