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.
Athlon 64 X2 4200+, UT nF4 Ultra-D, 4GB RAM, 128GB m4 SSD, 500GB HD, 9600GT 512MB, Intel 1000PT, Corsair TX650V2, Customizer 105 USB Blk. kubuntu 11.10 x64.
HP G62t, Pentium Dual-Core P6100, 3GB RAM, 320GB HD1, Intel HD IGP, kubuntu 11.10 x64, Win 7 x64.