Wow I love these threads (give me old procs over a tablet or smartphone anyday!)
I'm assuming that you don't have to have owned them to make it on the list, but here's my picks:
Zilog Z80: Very capable little processor, started playing around with them, nicer than the 6502 imho.
Motorola 68030: A personal favorite, I had one of these as an expansion to my Amiga, mine ran at 50MHz and was noticeably faster.
Intel 80386: A land mark CPU, at least in the PC space. The first x86 CPU in which is was possible to move away from the horrid segmentation and into a relatively clean 32-bit paged environment. Sadly the 386 was obsolete by the time proper operating systems were available for that environment.
Motorola 68060: The first processor I really wanted, but could never afford, the top end processor in Amiga accelerators at the time. I now have one in my current A1200 (again at 50MHz) and it feels very fast.
Alpha 21264: Never owned one of course but in its day it was the one to beat afaik. I do have a 21164PC to play around with though.
Athlon Classic: The first CPU to seriously content with Intel's offerings in a decisive way, with the competition started here, we're paying decent prices for Intel CPUs here, a landmark moment.
Athlon64: Another landmark, pushing PCs to 64-bits.
If I had to pick a favorite, it would be the Athlon Classic.
BTW Interesting that the Cyrix 6x86 hadn't been mentioned, Red Hill rave about it.
Intel Core i7 4790K, Z97, 16GB RAM, 128GB m4 SSD, 480GB M500 SSD, 500GB WD Vel, Intel HD Gfx4600, Corsair HX650, Fedora 21 x64.
HP G62t, Intel Pentium Dual-Core P6100, 3GB RAM, 180GB SSD, Intel HD IGP, Fedora 21 x64, Win 7 x64.