frat rankings?

Only three you should consider are Kappa Sig, Sigma Chi and Pi Kapp.

Kappa Sig- Huge chapter but bid just about anyone. Lots of parties at their annexes, pretty clean guys. Not very traditional but new house looks cool. Girls like them because they are pretty well put together and don't get in a lot of trouble.

Pi Kapp- mix of cool guys and weird dudes. Huge chapter and lots of parties. Mix with sororities pretty well because everyone knows pi kapp. Not very traditional but very well known because of their size.

Sigma Chi- Middle Size chapter, older guys know what's up, but younger guys not so much. A year after their huge membership review and still a presence on campus. Huge parties but no frat house. Very Traditional

The Greek scene at CSU isn't super strong, but you'll see a lot of people say on this site "if you don't rush fraternity X,Y, and Z it isn't worth it". Unless these people have been in every fraternity, I'd take statements like that with a grain of salt.

The "best" fraternity for you is literally up to you.

Maybe you want to join a fraternity with a house, or maybe you want a smaller brotherhood.

Don't limit yourself. Get out of your dorm, go meet some guys, and make your own opinions.