I've been to the following Big 12 schools.
Baylor
KU
Nebraska
Oklahoma State
Texas A&M
Oklahoma
Texas
I like the Oklahoma schools for the average size of the fraternities. You can tell that greek life is important when you have monster houses and more then just a couple large chapters.
Texas, I don't have much experience here so I'll reserve judgement.
A&M and Baylor
Nebraska is pretty badass in my opinion. Nice houses, and pretty big chapters.
KU has a lot of history. Can't tell you much as it was a while ago, but I had a blast in Lawrence, the houses were ginormous.