LSU.
Everyone keeps saying this, but they haven’t played in a while, LSU has bigger rivals in the SEC now, to make it a good rivalry A&M would have to be competitive in the SEC west (which isn’t going to be for a while, assuming they’re not just another Arkansas). Could they turn it into a good rivalry? Sure. Would it be like the Texas-A&M rivalry or the Texas-OU rivalry? I’m not sold.