In order for me to decide who was the best foil for The Rock, I'd have to look at both consistency and talent (wrestling and non-wrestling moments). That being said, as far as consistency goes, three of the mentioned (Triple H, Stone Cold, Chris Jericho) were highest in frequency.
I've always thought that SCSA was only overrated when in overall comparison to The Rock. There was nothing that he could do that The Rock couldn't do as well or better. SCSA was never in my opinion "great" on the mic. I feel he was good at being direct and straight to the point...In other words, he was good at "being himself" on the mic. In the SCSA character, he in many ways is being himself and it shows when he doing unscripted interviews and is STILL very blatant, direct, and makes those comical southern-sayings.
Chris Jericho on the other hand is in character much of the time and can clearly top SCSA on the mic. He can also do everything SCSA can do in ring but with slightly less presence and excitement in-ring than a match involving SCSA or The Rock.
Triple H has always been a favorite of mine but to be completely fair, I've always felt like he's a few steps below The Rock and SCSA in delivery on the mic and excitement in the ring. Triple H is original in that he has these strange identity crisis. He'll change his personality while still within the Triple H character. It's sort of a badly written battle between being a brute and being an asshole. Sometimes, he thinks he's as badass SCSA and at other times he thinks he's Attitude Era jokester, Edge. And in either personality, he never quite sells it as well as those two.
Overall Stone Cold wins my vote simply because he has the frequency with The Rock, the talent, and for the exception of being great on the mic (being only "good" on the mic is no insult to him), he can do EVERYTHING else The Rock can do.