You won't find the best transcon premium flying from Boston. The best you'll do from there is domestic "first class". FYI "first class" is a term that's misleadingly used for typical, two-cabin domestic USA flights with 2 x 2 seating in the front cabin, as opposed to the typical 3 x 3 in economy. Domestic first leg room is about 38" (as opposed to about 31-32" in economy). Really more like intra-European business class, though legroom is a bit better.
So you'll want to fly JFK to either SFO or LAX for true premium. A number of airlines have much roomier business and first class cabins on those flights. I agree with CDKing that the best possible seat might be the JetBlue suites being launched this summer. (Sixteen seats in the forward cabin, with 12 of them roomy 2 x 2 far better than ordinary domestic F and 4 of them being the 1 x 1 suites.) This post provides some useful information on that option, which looks to be relatively reasonably priced right now.
http://boardingarea.com/frequentlyfl...ghts-from-499/
In addition, though, AA is launching very nice, new, three-cabin planes between LAX and JFK in early January and between SFO and JFK in early March. First class will be 1 x 1 in seats very much like its new, lie-flat international business class. But domestic business class on these routes will be nice and roomy also.
UA, DL and Virgin have somewhat similar options, but JetBlue and AA look to be the best.