My vote is the AAC. They already dominate the G5 access to NY6. Boise wants out of the MWC. Bring them +1 more. Zags toyed with leaving the WCC, bring them as well.
With top teams in Football like: Cinci, Houston, UCF, BYU, Boise, SMU etc (I also like Navy as a conference mate)
and
top teams in Basketball like: Gonzaga, Houston, BYU, Wichita St, Memphis, Cinci etc
This takes what is already a very good conference and raises them up a level. The AAC by the numbers has floated closer to and sometimes above divisions of the ACC and Pac12, than to the MWC and other G5. With this change, the gap widens and BYU is in a better spot than they have probably ever been in the modern era of sports.
I'd still love to be in the Big12, but with my acceptance that this won't ever happen, the above scenario is the best case for BYU.