US/DL/NW/CO give unlimited North American upgrades (non-Hawaii) to elite members, though the policies differ slightly by airline. You'll have to read through the terms and conditions of each airline for more data.

UA and AA require all but the top elite members to earn and burn upgrade certificates.

WN and B6 have no upgrade. I can't speak to the terms about FL's program, or TZ's new program that's coming out, but those are really more niche markets that don't apply to a Boston flier.