The original Halo (Halo (1) if you will) was genre defining, amazing, original, fun. Quite simply it was the best shooter to hit a console since Goldeneye 64. Vehicles, Co-Op mode. Addicting maps.
Halo 2 was fun, added some things like double weapons, but overall still felt like just a cheap addition to the series.
Halo 3 was basically an Xbox 360 version of the same thing.
Halo has been living off of the fan base it acquired from the original Halo title.
Resistance 1 was a solid, fun title. I would have given it a solid 86 - 90 rated single player. It's multi-player was not fun in the slightest however. Resistance 1 was a fun game overall. It was an initial PS3 release title. The graphics were pretty solid (Definitely better than Halo).
For a new OP, Release title, Resistance 1 was solid.
Resistance 2, drastically improved the Online mode. 30 vs 30 Online Team Death Match is unparalleled by any game on any system. The online cooperative mode is absolutely a lot of fun. The guns are all well balanced (To the guy that said 'he doubts R2 is as balanced as Halo').
The Single Player campaign in Resistance 2 is epic. Absolutely epic. There's quite a few times during the game where you feel like you are watching the movie Independence Day and you are taking on the entire Chimera force.
Resistance 2 is better than Halo 3 because of several factors. The online for R2 closed the gap with Halo. R2 Online mode has it's benefits and Halo 3's Online has it's benefits. They are comparable.
But Resistance 2' Single Player campaign blow Halo away. Much better.
And if you want to go by "Sales Numbers" and "Reviews" of games to determine which game you enjoy the most or more....You might as well go pick up a copy of Wii Sports to live off of, and Assassins Creed.