As far as flights and US based frequent flyer programs, United has the most availability and most number or partners. I'd say that United miles are underrated at the moment given that they have the most generous routing rules out of any of the US based airlines. As long as you're flying from the east coast (or willing to route through the east coast), you can find a good amount of availability on Lufthansa, Swiss, Austrian, Turkish, United, LOT, or Brussels (or TAP Portugal and SAS).
Last year, we were able to fly Austrian from EWR-VIE, Turkish VIE-IST, and Turkish/Air Canada IST-YYZ-PHL all one ticket. We could have taken it further by buying a ticket from IST to another city and using the second jaw we had available to us to visit a third city but you know... time constraints.
AA would be a decent option were it not for the fact that most available flights using AA miles are on British Airways with their ridiculous fuel surcharges. Air Berlin and FinnAir are options with AA miles but you don't have the plethora of choices that you do with Star Alliance.
Delta would be a decent option as well except that their main partners save most of their award space for their own members and Delta's prices for their own flights are ridiculous, along with not so generous routing rules.