Ira / Highland is still a good rivalry. Ira / Hermleigh is a good one too, battle for Scurry County. Both are far from the best in Sixman though but still lots of history between those teams. I would think 60+ years of games constitutes a good rivalry in my opinion. There is still heavy emotions when Ira plays both of those teams every year. If I'm not mistaking, Ira has not lost a regular season (or home game) since the loss to Highland in 08', something like 45-47 consecutive regular season / home game wins I think (don't quote me on that).
Gordon / Strawn and Ropes / Meadow are the ones that come to mind when you say the "best" rivalry.