Is it the best Mexican place in Boston? Very likely but I have no proof. What I have a proof of is thousands of burritos and tacos eaten here over years. I was never disappointed if only in myself for overeating.
I find fish tacos and burritos the best.
The staff is accommodating to reasonable requests (gluten free, etc) but don't expect a bespoke cooking. Yes burritos are overflowing and not convenient to eat on public transit (which by the way you shouldn't do anyway) but you can always get a burrito bowl.
Parking can be a challenge if parallel parking is a challenge for you, but if one is willing to walk there are more spots further along Comm Ave.