Really, really tough choice for me since I hold Resistance series so dearly in my heart.
R:FoM along with MGS 4 were the main reasons for me to go with PS3 this gen and get back to console gaming after several years overall.
I have a soft spot for games with alternative timeline and in which your protagonists or humanity isn't on the winning side (it did not fulfill the second point entirely). Also this game had a WW2 sort of atmosphere, and I love those type of shooters. England setting was definitely the most atmospheric in the entire series and it had the best supporting characters apart from R:Retribution. Mechanics and technologically though it was subpar to the sequels.
R2 was decent, it also had fun 8 player coop. Overall it was the low point of the series for me, not going to point why, those points are well known.
R3 had a quite amazing SP campaign, the dire situation of US in 1957 was really greatly portrayed, the art design was absolutely great. Story was maybe the best in the series. Definitely the high point of the series mechanics and technologically speaking, which is expected (great 3D mode, try it) although it suffered some AI bugs.
Looks like the clear winner, right? Two things though. And spoilers ahead (although the game is over 2 years on the market and 20$ or less for quite some time so you have only yourself to blame if you haven't played it)
First, Insomniac left out 4 years in which we don't know much what happened. They didn't explain why Earth is/was not in the Solar System anymore at the end of R2? Why there were 2 planets/stars on the skyline.
And second thing, the worst thing Insomniac did (because of what I will be their lifetime hater) and Sony did not disagree on, are those three minutes after the actual game finished. For three fucking minutes of "post cards", "photos" and radio broadcasts that talked about humanity winning the Chimeran War and rebuilding the society, they have practically destroyed any chance for a future direct sequel, ending the franchise as we know and love it. For that fuck you Insomniac Games.