I mainly fly Delta as it is the largest carrier at my main airports. However, I credit my flights between Mileage Plan and Flying Blue.

As a rule of thumb, all of my international travel, as well as my domestic first flights, are credited to Flying Blue. My domestic coach tickets are credited to AS. For my use, I can get excellent value from my Flying Blue miles for transatlantic promo awards. I use my AS miles for redemptions on multiple OneWorld carriers (including AA, BA, QF, and LAN), all of which have had good availability in my experience.

While I do end up flying Delta for a large portion of my yearly revenue flights, I always take Alaska, KLM or Air France when possible. Based on the comments from other posters in the Delta section, I know that I'm not alone.