I think it depends on a few factors:
1) Are you loyal to an airline/do you live in a market dominated by 1-2 airlines (I presume you live nearest to Logan).
2) Do you fly a lot or a little? By yourself or with family?
3) Do you want status with an airline, or just points to reduce the cost of future travel?
Personally I have the Chase Sapphire Reserve, which can a) transfer points to a variety of airlines, and b) has points which can be used to purchase points through airlines, most of the time at better rates than those airline’s points. It has a steep fee, however, of $550/year, with only barely enough other perks (various statement credits) to make it worth keeping if you don’t travel enough. I’ve travelled much less post-Covid, and unless that changes this year for me, it’s my last with this card.
I also this year dipped into the Amex Delta card, mostly due to a nice signup bonus of 90k miles. It has a $250 annual fee, but comes with a free continental US companion ticket, which pays for the card itself with me, since we travel to my parents on Delta at least once per year, and those tickets always run more than $250. Delta recently nerfed its miles program, however, so the card is unlikely to help me get status.
When Baltimore was my home airport, I used to cycle through the Southwest card from Chase every other year. Around February it’s signup bonus is a year of companion pass (plus 30-50k points), so I’d get it for a year, cancel, and then get it the following year (you can’t get the welcome bonus more than once every 24 months). If Southwest gets you where you want to go, I think that card, in February, is the best deal if you’ve typically got a companion. The signup bonus saved us about $4k in tickets the last time we got it.