I'm a senior at CU planning to walk this May so I can give genuin help.
It's a very unique school whether people want to admit it or not. There are very few schools where the Greek scene, house party scene, and bar scene are all equally as big. Being in a frat is not a must at CU, but not a bad choice either. From my experience, your dorm buddies will always be close friends, from there you choose where to go.
All the frats are different and will give people different impressions. Rush them all and see what you like, but rush is one giant man flirting session, and everyone admits it. Talk to your friends, see what you like.
I don't like frats here but I can say Sigma Pi had the most skiers when I was a freshman and checked them out 3 years ago. But for real, since I've been old enough, I haven't been to a house party in a very long time. I spend most of my nights on Pearl.