I went to the game in the Phog this year and it was loud, but it wasn't any better than GIA in full force.
I'd definitely say the KU game in Stillwater was much louder than the Phog was in our game up there. My ears were ringing in GIA and the noise never got to that level in the Phog, even though I sat in the student section. I was way up at the top of GIA too.
Now, that said, the Phog has a MUCH more consistent atmosphere. GIA's peaks are better, but it has a lot more low points than the Phog. KU fans bring it every game, when OSU fans just seem to show up for big games. Granted every game for KU is usually big because they are always good.
Also, both games the home team lost and trailed for the majority of the game, so I'd say it is a very good comparison on equal footing.
I don't think Hilton is on the same level from what I heard tonight, but it is VERY difficult to judge things like this on TV. I think you could currently rank the Big XII arenas like this...
1st Tier
1. OSU / KU
2nd Tier
3. ISU
4. KSU
5. Baylor
6. UT
3rd Tier
7. OU / WVU
4th Tier
9. TT / TCU
OU seems to be a tough place to play for the Cowboys for some reason, but the atmosphere just isn't that intimidating. Kind of like UT in football. Tons of fans, but not that loud, and not very close to the field / court.