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.