Very interesting thread... Have always wondered myself.
I've only been playing since July 2013, and my first real rewards card was Dec. 2013 (that inquiry fell off a few days ago, yay!) so I haven't churned nearly as much as many of you. . This year was big for me so I'll be faced with The decision after the New Year. Some on the list include what I think I'll pay the AF on.
Card -- Annual Fee -- Rationale
CSP -- $95 -- haven't been able to open again bc 5/24, so need for transfer ability
Prestige -- $350 [citigold checking] -- with the $250 Credit, and anticipation of using the 4th night free benefit this will be a tough decision. I also fly a lot of AA domestic awards and the lounge access makes flying a lot more comfortable.
Marriott -- 85?-- no the Cert isn't worth the AF but I have an award booking with points from two accounts. May need to keep open until I can transfer as they limit how many points you can transfer each year.