Best Program: US Airways
Best Feature: Unlimited upgrades for Chairman's Preferred & companion.
Con: Lack of partners. Non-automated upgrades (you have to call and ask.) Lousy coffee. Only 2 SWUs for top tier (the 2 companion certs don't count).
Other notable positives: "Personal Liason's" are superb. Great treatment from staff all around -- ticket counter, gate, flight attendants etc.
CO's Best Feature: Automatic upgrades. Great (domestic) 1st class menu is a close second.
CO Worst: You have to fly on an old, dark and gloomy Boeing fleet -- even the brand new planes feel like they've been left out in the rain somewhere near Detroit for 30 years. The most uncomfortable 1st class seats in the US.
NW Best: Automatic upgrades.
NW Worst: Nothing notable springs to mind.