Utah. They would have brought in many of the top LDS recruits in football and basketball from the country. It would have given the Pac-12 a team which had a chance to compete for the college playoff every once in a while.
Instead they got Utah which has made the best for their situation but will never be able to get the recruits to make a real run. Utah is just not big enough state. Utah as a program has on paper their best team ever but still not enough. They have a great coach with a great system but do not have the reach necessary to take it to the next level.
While I say this I recognize many of the other Pac-12 programs have benefited from BYU not in the Pac-12. They've done this by stealing alot of good LDS recruits who don't want to make the decision between BYU and Utah. I don't blame those kids. You pick Utah so you can play in the Pac-12 but you almost betray your religion. So, you leave the State to avoid the controversy.
Pac-12 divided the state of Utah and now Utah has two middle of the road teams. Neither will be able to make a run for the college playoff. Worst for the Pac-12 is their brand is dying. All good football is played in the south and East right now.