Great discussion. One of my favorite beer-drinking-while-talking-sports topics, actually. Here's an outsider's take:
Why CSU is CU's top rival today. The two schools have comparable recruiting areas, athletic budgets (closer than CU and Oregon), academic standards (don't flame me for that -- CU and CSU are closer than CU and Utah in at least
one ranking) and alumni bases with common backgrounds. The two schools have played more times than either has played anyone else. Most important, IMO, to the whole "rivalry thing" is the recruiting -- bottom line is that CU and CSU are fighting for the in-state talent and to those kids, the annual game is important. Quotes from players at both schools seems to indicate they see it as the big rivalry (in my book, the players' opinions count more than fans).
Arguments why CSU might not be considered CU's top rival. Games between the schools haven't mattered significantly as long as I've been following college football. To my knowledge, the Rocky Mountain Showdown has never been for a conference championship, or for a trip to a bowl, or anybody's "National Championship". CU has dominated the series historically, so it's at best a RINO (Rivalry In Name Only). Playing the game in the first week of the year at a neutral site full of empty seats doesn't help -- part of the fun of rivalries is beating your rival at their house and spoiling the day for all the recruits they invited to the game. To be a rival, one could argue that being in the same conference is a requirement.
My Conclusion: CU's chief rival in athletics is the Denver Broncos. Sorry, but the three months I've been in the area has done little to refute the stereotype that Coloradans are "pro-sports" fans, not college fans. CU grads seem to get more pumped up for an early season non-divisional Bronco's game than a Pac-12 CU game. Except for
maybe within Boulder city limits, I see more licensed merchandise for Broncos/Rockies/Nuggets/Avs than CU by almost 2:1 (no hard numbers, just casual observation). Small data set, but I work with a number of CU and CSU grads -- none of them care about college sports one-tenth as much as they care about the pros. To each his own: if people would rather spend $125/ticket to watch a bunch of millionaires play each other instead of supporting student-athletes from their alma matter, God bless them. I also note the number of comments on the hoops board about poor attendance being blamed on concurrent Broncos games (many comments seemed to imply that fans were choosing to watch the Broncos on TV rather than the Buffs in person -- there is no help for that guy). In an office of 70 people, there's only one other dude that follows CU athletics (and he's not alumni either) -- I don't like it, but my work productivity has increased with the lack of interesting sports conversation at the water cooler.
Not that most of you care, but this subject comes up with Virginia Tech as well. Popular belief, supported by a contingent of VT fans, is that UVA is our top rival. However, there are sizable number of Hokie alums who see Miami in that role, particularly those from my era, who remember VT and Miami playing for Big East championships, playing for a slot in the BCS title game, playing for a top 5 ranking and eventually coming to the ACC with us and continuing play for high stakes. Until we joined the ACC in 2004, the game against UVA rarely had anything significant at stake (multiple years I recall reading analysis around Thanksgiving that VT's bowl destination was more defendant on some intra-Big East game the same weekend than the VT/UVA game itself). More so, the VT/UVA game has traditionally been played the Saturday after Thanksgiving, while most students are out of town and only the die-hard student football fans would plan their fall breaks around the game -- I know that's the 'traditional' day for rivalry games but I don't necessarily agree its a good date for student-fans.
Edit: realized in the academic ranking link, Utah and CSU are tied, so their the same "distance" from CU by that measure. CSU is listed higher on the page and I didn't look closely enough.