PSA: Do your research before you buy the games. Red Faction Guerrilla is the open world DESTROY-ALL-THE-THINGS game. Red Faction: Armageddon is a linear mission based go-here-kill-things-pick-up-battery-you-win-repeat game. I don't know anything about Red Faction or Red Faction II.
Red faction guerrilla is the best game ever. You can mod the stuff out of it and just destroy everything. Yes, every single building in the game is destructible with great physics. Remove all the enemies if you want or shoot the forget out of them.
It has an amazing physics engine combined with a /huge/ open world where you can run around everywhere, or take a car with fancy car physics if the way is too far, on the ride there run over other cars, people, or entire buildings. Place charges on car, crash the car into a building, jump out, BOOOOOM!
It's just amazing, I can't stop playing this...
I also got armaggeddon, played through the campaign but it's just a pile of crap compared to guerrilla. Everyone loves the amazing open world. WHY DO YOU TAKE IT OUT?! *table flip
Sometimes developers are just major idiots. Of course that stuff sells bad! And what do you do? Put back the open world? NO! END THE WHOLE SERIES!
Didn't get the older ones because no physics destruction engine.