The Solti's Ring still is the benchmark, because the casting is marvelous, the orchestral playing is super, Solti has the solid approach, and it is stereo with the best sound. But for me, this cycle should only the first ring, because Solti don't offer so much insightful interpretation. The tempo is moderate, phrasing is ordinary, all his goal is reach the climax, such as Ho-jo-to-ho. This approach works best in Siegfried, but not great IMO in Die Walküre and Götterdämmerung. I think Keilberth Ring suffers from the similar mistake.
For me, the three best Ring is from Furtwängler, Knappertsbusch and Karajan. The Furtwängler Ring has the bad sound, but the interpretation is super, he can easily reach the power like Solti, but he produce highly passionate in the slow scene. Karajan produce the beauty and melody of the Ring better than most conductor,even if the cast is not strong, that only because he want it. He don't want Bruhilde like Nilsson, full of power,passion but lack the love, that why he choose Dernesch and Crespin. Knappertbusch is hard to describe, but for me it is spiritual, especially in Götterdämmerung
For the modern recording, I like Boulez and Thielemann best because they both offer the unconventional approach to the Ring