Well, here's how I see it:
The iMac certainly started things off. If it weren't for the iMac, Apple may very well have either been sold off or shut down completely. 1997 had been dire for Apple. People may forgot now, or may be too young to even remember, but it was massive in '98 and '99. Other PC vendors tried to copy it, products completely unrelated to computers stole its design and color scheme. People actually spoke about Apple again, and in a positive way! Amazing...
That being said, I don't think the iMac was enough to keep the momentum going. Yes, it was a huge hit and got Apple tons of media attention, but two very important things followed that allowed Apple to keep going:
1. Mac OS X. This made the Macintosh a viable platform again. If Mac OS X had not been released, all these people buying iMacs would have just gone back to PCs for their next purchase.
2. The iPod. A true category killer, this became the de facto digital music player - as has been mentioned. Its also lead to people taking a closer look at Apple's computer offerings, and making the Apple brand one of the most recognized in the world.
Still, even with the iPod, I think it's Mac OS X that we keep going back to. Someone buys an iPod, thinks it's great, take a look at a Mac and thinks it's equally great. Ends up buying a MacBook or iMac and now they're a Mac user. This scenario would play out very differently if Mac OS X wasn't as well-made as it is, and has been for many versions now.
The iPhone simply builds on what the iPod started. And remember - it's built on OS X!
And the Intel Macs have helped a lot in pushing the Mac platform to both consumers and businesses, but I think that yet again we find ourselves thanking Mac OS X for convincing people to turn to Apple in the end.
It's very close between the iMac, iPod and Mac OS X, but I think when it all comes down to it, the operating system is the constant that's been helping Apple out through it all.