I absolutely adored the first, but felt the second completely fucked up everything that was good about it.
The first had a brilliantly oppressive atmosphere. The graphics where fittingly old timey, with a perfectly simple color palette.
The plot was wonderfully mysterious, giving the enemies a real sense of threat, as you had no idea what might be coming next.
I loved how the story was told through what felt like after action reports, with a nearly silent main character.
Finally, I've never seen a modern game that can claim the astounding variety of weapons and enemies Resistance 1 could. Swarms of crawly fuckers to regular soldiers to massive hulking cannon platforms to giant dogs to HUGE fucking spiders PLUS 2 types of vehicles with their own whole thing. In terms of weapons, from the Splitter to the Auger to the Carbine, every single one was interesting, fun to use and useful in its own unique situation. The grenades even deserve special mention. The grenades that deployed gas and lit it for a giant fireball and the grenades that literally deployed a field that turned every shot toward its shooter are my favorite.
2 just made the aesthetic identical to over other bright looking game, made the protagonist talk his ass off and made the guns all feel weak, while only allowing you to carry 2 of them.
3 was a bit better with the infinite weapons and whatnot, but it still suffered badly as a result of the aesthetic and narrative changes.
I could gush about 1 some more, but I think you get the idea. One of my favorite games of all time.