Jay Miner's team doing their own chips (Antic, Pokey, C/GTIA) instead of assembling a machine off the shelf. It was a truly fun machine to program. It was amazing what could be done on the Atari machines that couldn't be done on the Apple ][.
I have a love of both. Atari was a chip playground. And the SIO system was very forward looking for the time. Look at FujiNet for an example. Old software working across the Internet!
The Apple was a tank, fast workstation. (Assuming one boosted the 1Mhz CPU.) I had 4Mhz at one point then.
My //e can run at 16Mhz. Crazy fast. Makes it useful today.
Atari machines can get a high Mhz upgrade, and if one wants to do it, a 6809!
One of the things I remember most about the Atari 8 bit family was the superior graphics and sound as compared to the early generation of dos-based PCs that came years later. The haunting melody from the splash screen of 7 Cities of Gold was classic. And it still blows me away to see what the Atari 8-bit was capable of with games like Alternate Reality: The City!