I haven't owned all of these but:
- 233mhz pentium MMX. These were incredibly common as they were the last Intel chips you could get on a lot of socket 7 boards. A lot of people entered the new millenium on a 233 PI (myself included), and ran it for a few years into the new millenium too.
- 100mhz FSB PII chips. A lot of people skipped the 66mhz fsb chips and ended up with PIII 350/400/450 chips (especially the 400). I can't even count how many PII 400 boxes I've come across. Sadly I didn't own one, I waited all the way for a 600mhz PIII coppermine
- The Original Slot A Athlon's. AMD finally had a chip that could beat Intel's chips at the same clock speed (for a little while). The two were neck and neck performance wise.
- The coppermine PIII's. AMD couldn't answer this threat very well until they went to socketed chips again.
- Northwood Pentium IV chips. Nobody owned the Willamettes (well I owned one), but the Northwoods introduced some pretty excessive memory bandwidth for the time.
- The Athlon 64, from the 754 pin socket all the way up through the socket 939 X2 chips. Intel had chips that could compete with them, but they couldn't compete well. When Intel came out with the Pentium D they got a reprieve though, as the X2's were expensive by comparison.
- The core 2. The architecture may have been tweaked a bit, but it's been king of the hill performance wise since it's inception.
-6 core Phenom II chips. Intel should really sell a few 6 core chips for an affordable price. Also, those 12 core opterons are insanely awesome. Lower clock sped, but 12 cores, and support for up to quad channel DDR3.