Objectively speaking, BYU is the best single team for the Big XII to add. There is really no good reason they aren't in a P5 conference, as they've always been and always will be a relevant football program. After that you've got a bunch of teams that all bring something less to the table. I think Boise has the best current brand of all the remaining teams, but there are legitimate concerns about what happens when Boise isn't good at football (more than there were with TCU). At least TCU will always have proximity going for them if their football program cycles to the bottom.
Now from my non-objective fan view, the reason I honestly think CSU has a better chance to join the Big XII than UCF/Memphis/Cincinnati/etc, is the historical tie to the Denver/Colorado market and to provide a logical bridge/pair/rival with BYU (although the same could be said for Boise). Plus CSU did just commit to building a brand new stadium and are enjoying current success, so they look to be headed in a promising direction at least. This is honestly the only time I could ever possibly seeing CSU joining a P5 conference, because it is a perfect storm of having a good stadium, excitement around the program, and the Big XII in desperation mode to add two teams because they just got fucked by the playoff.
Now from my non-objective fan view, the reason I honestly think CSU has a better chance to join the Big XII than UCF/Memphis/Cincinnati/etc, is the historical tie to the Denver/Colorado market and to provide a logical bridge/pair/rival with BYU (although the same could be said for Boise). Plus CSU did just commit to building a brand new stadium and are enjoying current success, so they look to be headed in a promising direction at least. This is honestly the only time I could ever possibly seeing CSU joining a P5 conference, because it is a perfect storm of having a good stadium, excitement around the program, and the Big XII in desperation mode to add two teams because they just got fucked by the playoff.