I am a BOS-based flyer though live part of the year in Marin County and the summer outside of Montreal. I'm been concentrating my point accumulation on BA (I'm Gold for Life) because I have taken the BA day flight to London for over a decade 4 to 15 times a year and more recently JetBlue because it is good for BOS-SFO and generally for flights to Florida and Charleston SC, etc. Because I fly all over, I am usually Star Alliance Gold but to get it, I need to credit my flights to Turkish Airlines (used to do this with Aegean and before that AC, thanks to the good advice of Flyertalkers). The primary virtues of Star Gold are: 1) access to lounges in Canada, the US and elsewhere without having to pay; 2) occasional upgrades to first on AC. I usually fly business class for work but will take flights to Calgary on AC and appreciate the upgrades.

In the past, I have not flown SkyTeam unless I had to go to ATL or occasionally Paris. However, Delta is ramping up in Boston and I am flying to BOS-ATL-SP-ATL-SP later this month on Delta and then to London later on Virgin (just came back from London on Virgin, though I will say I didn't love the Upper Class seats). Given AA's apparently self-destructive strategy, I can imagine that I might choose to shift significant business to SkyTeam. Is there an airline on SkyTeam that makes it easier to get useful elite status (e.g., gets in lounges, maybe upgrades) like I get on Star Alliance via Turkish?

I fly to London, Toronto, Calgary, NY, Sao Paulo and San Francisco regularly but also to Rome/Venice, Stockholm, Frankfurt, Tel Aviv, Mexico City, every once in a while and on occasion to almost everywhere (Sydney, Singapore,Vietnam, Croatia, Prague, Vienna, etc.). Usually business class for work but premium economy for vacations. In the past, I've avoided Air France and Alitalia.

Thanks for any advice.

What is your criteria for best airline?
Best airline and best frequent flyer program are not necessarily the same.
Generally you are better served by the airline you fly most.
Getting status with revenue based ffp's seem to be harder.

Lounge access is usually by class of travel or mid-top tier status. USA based ffp's are unusual in that mid-top tier status does not give lounge access on USA domestic flights or USA to/from Mexico, Canada, or Caribbean flights (but check the rules).

Getting an upgrade on any airline other the airlines of your ffp is a tough ask/hope.

Skyteam:- Which SkyTeam program should I sign up for?

Be mindful of expiry Miles/Points that Do and Don't Expire. Click to open the wiki.

The grass in not always greener on the other side

If you are not a ST elite of another airline than DL you wont have access to DL lounges except if you are on international trip

If you fly DL as an ST elite of another airline than DL you probably have more chances to date beyonce than being upgraded

So from BOS you have KL , DL , AF , AZ and KE , which is pretty wide choice

Then UX or FB can be suitable for you , you should take a look . If you fly long haul in prem eco and bizness then FB might be easier

I'd say Delta. Virgin Atlantic isn't part of skyteam and neither is Westjet. However Delta is a partner with both which helps you maximize the benefits.

Korean is fairly easy to qualify for lounge access due to its 2 year cycle. However, for lounge access alone which you could get with an Amex card. The Amex reserve can get you a lot of mqm with spend and includes delta lounge access.

Being Boston based, I'd suggest a priority pass credit card for lounge access. Good restaurant options in B and C. Lounges in C and E. What's missing is A and Delta.

Thanks for your advice. Just back from SP and Stockholm/London.

I'm hoping that there is an airline in the SkyTeam alliance that makes it easier to reach sufficiently elite status as to get US lounge access (similar to the benefit I get from the relatively easy route to Star Gold that is available by crediting miles to Turkish). I do have Plat Amex and Priority Plus..

Go to know on the upgrades. Sometimes as a StarGold through Turkish, I get upgraded on AC. I don't think I get any upgrades these days on OneWorld.

Then only delta fits your requirements if you will travel in us domestic routes . If not then check FB or UX

then probably AC flights are good to fly when crediting miles to TK

You mean priority pass ??

UX will likely leave ST soon, so definitely not a good plan.

In this moment, alliance-wise and in a medium-long term perspective, I would go for FB program: it is supported by AF and KL and still ensure good benefits in Europe even with UX (leaving ST at some moment) and AZ (unstable but still operating and unlikely disappearing...). DL however can be a good option if you mostly fly in/to/from US. But DL may need less ST than AFKL and so they could even leave it at some time while preserving other JV. so it could be difficult to use their miles in services of other airlines. Of course, just my 2 cents.
As said before, I agree you should choose the program based on where you fly the most.