Okay, I'll take a stab at this.
1. The original Macintosh.
Without this, we'd have no OS X, no iPod, no apps, nothing. The Macintosh was the base that Apple built the company on.
2. Mac OS X/NeXTSTEP.
There's no doubt about it, OS X gave Apple a new life. Without OS X and the leadership of Steve Jobs following the acquisition of NeXT Apple would've sunk with a flailing user base and a struggling OS. The OS X/Jobs combination made way for the definitive Apple products we all know and love; without Jobs and OS X, we'd have no iMac or iPhone/iPod touch.
3. The iPod.
The iPod brought Apple to the forefront of popular culture right off the back of a breakthrough OS. This one product pushed demand for Apple products so high that it was undoubtably one of the main factors in Apple's dive into retail marketing. With the iPod available for a good 'ole hands-on in Apple's own stores the rest of the line pretty much advertised itself. The iPod is much more than just a media device. It's a fashion accessory, a billboard, a 3rd party development platform and a synonym for digital music.
4. Apple Retail Stores
I'm going to class the Retail Stores as a product because they are, albeit a free one. In the six years of their existence Apple's Retail Stores have become renowned for their superior design and unique shopping experience. How many other computer stores can you go in, check your mail, see's whose online on AIM, edit a movie and upload it to YouTube while browsing MacRumors and listening to music in iTunes? Now how many of those stores let you test drive every single product they have on display along with get your own product troubleshooted by people who know what they're on about? And how many of those stores offer these services for free? The one and only Apple Retail Store. Consumers have realised that they want to shop the Apple way and because no one else offers this kind of service, they buy from Apple.
That's what I think is most important to Apple. The above four products target specific levels of consumer relation to provide an experience quite unlike that of any other company, and when they're combined, we can see how they all come together and build a "house" of sorts. The Macintosh is the foundation for everything, OS X built on foundation to give it walls and a roof, the iPod furnished it and the Retail Stores are the welcome mat.