I'm fond of the MOS 6502. It was really a breakthrough at the time for making CPUs affordable. It follows a "keep it simple" philosophy, one could argue that it's a RISC - but this was before "RISC" had become a marketing buzzword and so it doesn't meet the modern interpretation of that term. "RISC" processors are thought to always have tons of registers, and the 6502 has very few, but that's just a result of what made sense at the time. The bottom line is, the 6502 succeeded at it's job by keeping things simple, just like any "RISC" design.
It typically runs slower clocks than a Z80, but gets more work done per clock, so it was a good competitor against it.
There are 2 processors that I've programmed in assembly. One is the x86 (mostly in the days when we had a Cyrix 6x86 and a 486 in the basement), and the other is the 6502. I actually learned the x86 first. Some years ago, out of hobby interest I downloaded the documentation for the 6502 and was pleased at how simple it is. It's a nice CPU to deal with, everything about it makes sense. Back in the 70s they couldn't afford to overcomplicate it.
For x86 processors, I think I really like the Coppermine. I never owned one when they were new. But in more recent times I've used them a lot and been impressed at how cool and low power they are. The low end coppermines are cheap and common as dirt, and when they are adequate they are very cheap to run. I run some small stuff 24/7 on a headless Compaq iPaq with a Coppermine 550/100, and that whole running system uses 27W. With the original Mendocino Celeron it was in the 30s.
I've also enjoyed Coppermine based 440BX desktop machines. I like the motherboards they're used with, but that's straying from the topic I suppose. At one time, I think every computer I had built for members of my family, as well as myself, were 440BX based with Coppermine CPUs. I just love that platform.
For what I'd call modern CPUs, I like the K8. I really admired these when they came out, but didn't own one until recently. The NUMA architecture is cool, and the power management capabilities are a big leap forward from anything mainstream that I saw before it.