Artwork: Chip Taylor IBM's first PC, announced on August 12, 1981, was far from the first personal computer--but when it arrived, there was near-universal agreement that it was likely to be a landmark machine. It was. And 25 years later, it still ranks among the most significant computers ever.

Like the IBM Personal Computer, Model 5150, the greatest systems have always had ambitions to boldly go where no computer has gone before. Without these innovative machines, the PC revolution would have been a lot less...well, revolutionary. So we decided to celebrate the IBM PC's 25th birthday by identifying the 25 PCs that have mattered most--from any manufacturer, and from any era.

No single characteristic makes a computer great. But we managed to boil down an array of winning qualities into four factors, all of which happen to begin with the letter I.

Innovation: Did the PC do anything that was genuinely new? Did it incorporate the latest technology?

Did the PC do anything that was genuinely new? Did it incorporate the latest technology? Impact: Was it widely imitated? Did it become part of the cultural zeitgeist?

Was it widely imitated? Did it become part of the cultural zeitgeist? Industrial design: Was it a looker? Did it have clever features that made using it a pleasure?

Was it a looker? Did it have clever features that made using it a pleasure? Intangibles: Was there anything else about it that set it apart from the same ol' same ol'?

Armed with this scale, we considered dozens of PCs--which meant that we also had to consider the question "What is a PC, exactly?" Ultimately we decided that a PC is anything that's recognizably a desktop or portable computer in design--or, alternatively, anything that runs an operating system originally created for desktops and laptops. After a lot of nostalgic debate, we selected our winners. Which systems we picked--and didn't pick--for our Top 25 may be controversial. If one of your favorites didn't make our roster, check out our list of 25 near-great PCs.

Just to drum up a little suspense, we'll reveal the Top 25 starting with number 25, and then work our way backward to the single greatest PC of all time. (Spoilsports can skip ahead to number 1; we won't be any the wiser. You can also jump to the complete list of our Top 25 picks.)

Ready?