I wouldn't want to see 24 without Jack, in the same way I wouldn't want to ever have seen Buffy the Vampire Slayer without Buffy. Some shows are completely lead-based, and this is one of them. Tony was awesome before he went evil, and Renee is awesome, but 24 and Jack are synonymous in my book.
Totally agree. And I think BRAGA/COTO ruined a good thing with Tony. There is NO WAY Tony would have done what he did. Jack lost his wife, and never went rogue. Tony was positioned to take the show over if Jack had left, sort of. I could buy 24 with Tony, not as much as I do with Jack, but at least we had history with Tony. They ruined him.
Renee, as you said, was awesome. But as the show's star? No. 24 is Jack Bauer, as you said. And with out him the show would just fade away.