Infrequent flier, but most likely to find themselves on *A: Aegean.
Infrequent flier, but most likely to find themselves on OW: BA Avios.
Infrequent flier, but most likely to find themselves on Skyteam: I truly have no idea. Probably not DL though.

Infrequent hotel guest with access to all of the U.S. credit cards: HHonors.
Infrequent hotel guest with no access to U.S. credit cards: Don't really know, but probably Priceline, local boutique hotels, or maybe something like Club Carlson thanks to crazy promos that can be fulfilled with 1 night stays. Probably some region-specific programs (Melia, etc.) that are good but don't get a lot of Flyertalk play.

Infrequent car renter: Probably a mix of the base Emerald Aisle (National) service and Priceline. Or research airport-by-airport to figure out where companies like Dollar or Budget have operations that are on-par with the majors thanks to combined rental facilities, kiosks, etc.