Go back far enough on here and you'll find lots of positive posts from me about Story's FF films. Not so much anymore. I actually bought both dvd's last year, sat down to watch them and... turned the first one off maybe halfway through. Whatever fondness I might have once had from them no longer applies. Right now I'd sandwich them around Green Lantern (FF2 > GL > FF)
So Guardians gets my vote. Ironically GotG is also a movie I grow a little less impressed with each time I watch it; it isn't bad by any means but it's cutesy nature has started to irk me. I hope I'm not turning into one of those dour, humor-hating nerds. Blech.