From a design standpoint, its a three way tie between the new iMac G5 (i may not think so in the future) the cube (the most innovative still) and the original iMac (the first designer computer). From a technological standpoint, I say the G5 shook up some stuff upon release, as did the iPod.
Oh that's an easy one! The ballpoint pens they used to send when you registered Apple products are amazing. They seem have a magical property that makes them impossible to lose* or break. I think that mine will outlive me.
* "Borrowed" for lengths of time, but eventually returned.
If your question is for fun then it's the iPod. If it is serious then obviously it's the 128K Macintosh (1984), no question about it. Without it we might still be in a DOS world without even a mouse to click with.
Design wise, I would go with the Cube, though most of the recent Macs are also beautiful, thanks to Apple's great design team. In terms of overall success, it's hard to argue with either the original iMacs or the iPods.
tie for three; sorry im indecisive:
lisa 1 (without it there would be no macintosh--a flop but it paved the way for g5's and everything we know and love about apple), the original ibook (cmiw one of the first consumer laptops, had airport which exploded into an industry standard and is present in just about any modern laptop worth it's weight in plastic), and last but not least: the newton messagepad (among the first pda's, had handwriting recognition, several years ahead of it's time). these products may not have been incredible but they showed apple innovation and paved the way for a lot of the modern computing products we use today.
Candy color Imacs. this was the best seller and saved the company but what did apple learn with this product? nothing,everything that made it great they walked away from. It was the biggest success of all time and everything that made it so great they have ignored. oh well so much for a imac thats a tick away from powermac performance. Now we get waterdowned imacs with not even the fastest cpu from last years powermac. Apple learned ( 0 ) in what made the original the best.