Airport Lounges
Airport lounge access can come from credit cards, status, points, or otherwise. A traditional lounge offers a place to get a meal, get free wifi, and be in a more comfortable and quiet (as long as it's not too busy) setting than the main airport.
The main lounge network is Priority Pass, and different premium credit cards (typically $400/yr or more annual fee) offer annual passes, while some cards of lower annual fees offer a limited number of passes. Likewise, airlines like United sometimes offer a limited # of passes on lower-tier cards (i.e. 2 passes on a $100ish annual fee card) while offering unlimited access for higher-tier cards. Unlike most Priority Pass lounges, typically with airline credit cards, you need to be flying on the airline or partners to take advantage of their lounge network offered by the credit card.
Best app for lounges:
The Lounge Buddy app is fantastic for lounges. Just be sure to input your credit cards and various statuses into it for it to work well for you. The rating system and access information as well as amenity information on it are fantastic.
Best card for single user for lounge access:
The best card for a single user depends highly on the person and the amount of flying they would do. With any card, especially one with an annual fee (& I can't think of a single card where you can get lounge access without an annual fee, unless you count cards with free credits with no annual fee varieties that could be used toward lounge access passes, i.e. the Penfed Pathfinder, where you'd be likely better off with other purchases rather than lounge purchases), it's important to be sure that you are getting positive expected value.
That said, an easily accessible card that's known as the best card for a single user is the Amex Platinum & Amex Business Platinum cards. They offer Priority Pass access (though non-traditional lounges like Priority Pass restaurants or Gameway in LAX are excluded), Delta lounge access if flying on Delta, Centurion lounge access, and some other Amex partner lounges access. If you were to consistently fly out of DCA though on American Airlines, it'd probably be the wrong card for you lounge-wise since you wouldn't have any lounge access. It's also a lot harder to break even on the annual fee than the Capital One Venture X is, that would be great for DCA since you could have access to 2 different restaurant-based lounges.
While you won't get access to Priority Pass restaurants with it, a card that offers access to Priority Pass and to a hotel chain (Hilton) when you stay at a hotel with a lounge is the $450/yr Hilton Honors Aspire credit card.
To get a credit card recommendation on which lounge card is best for you, go here.
Best cards for adding authorized users for lounge access:
Arguably the best credit card offering Priority Pass when considering authorized users is the Ritz Carlton Reserve card (offering supposedly unlimited free authorized users, each getting priority pass access) which can be product changed from the Marriott Bonvoy Boundless, or the Capital One Venture X, offering up to 4 authorized users, each with Priority Pass benefits, at no additional cost.
Arguably the best credit card specific to an airline when considering authorized users is the Citi AA Executive World Elite Mastercard, since it offers up to 10 authorized user cards with no additional annual fee per authorized user, each conferring lounge access.