Here's my brief take on the three major programs:
As you probably know Delta has the largest presence by far but their miles program is considered one of the worst by some people.

AA has very little presence and you'd have to connect nearly everywhere you go, even domestically but can be good for international redemptions if you plan ahead.

United is considered by many to be the most generous when redeeming miles within their Star Alliance network. I also feel like they have the most mile gaining options via credit cards because of Ultimate Rewards by Chase and also Chase's United card.