I've read almost all of this thread and registered to chime in to help out a bit.
First, the most important rivalry game that will absolutely not be touched under any circumstances (even if it were to mean TX and OU not joining the SEC) is the Auburn v Georgia game. I can GUARANTEE you this. It is the oldest rivalry, it is the first rivalry, it is the first game ever played between SEC teams. It has always and will always come first. None of Arky, SC, AtM, or Mizz would currently be in the SEC if it endangered that game being played annually. I can promise you AU and UGA would instantly vote against anyone joining the league if that game was in any jeopardy whatsoever.
Next and in order:
2. Florida/UGA
3. Auburn/Alabama
The rest are negotiable and this includes Alabama/Tennessee. Both fantasies for YEARS on the SEC rant have said fuck it on that game. Not all, but an inordinately large number that grows yearly. The hate isn't there anymore, it is forced and people do not watch or care about that game in large part. However, it is old and historic and above the best rivalry from expansion...
4. FL/ LSU
The SEC won't prioritize keeping FL/LSU but I promise you everyone wants to keep that one going.
Finally and probably about to be a thing of the past is the Tiger Bowl, AU/LSU. It's probably gone but it's been the 5th biggest, most watched rivalry since 2000.
One last bit of perspective/history. As a condition of the first SEC expansion with Arky/USCe, each SEC team was given 2 permanent rivalry games. For one reason only. To maintain the AU/FL rivalry. Until the second expansion which dropped the second annual game, AU and FL was the 3rd most played game in SEC history and probably the 4th biggest rivalry in the SEC behind AU/UGA, Bama/Tenn, AU/UGA (in that order at the time in 1992).