Agon eZ80
As the author of the original Agon Quark firmware (you linked to my Git) and long time Z80 developer, I've grown to like the eZ80. It has its quirks, the most annoying being that there is no straightforward way to access the top 8 bits of 24-bit registers without storing the register in memory first. Yet that not withstanding, being able to access more than 64K without paging is incredible, and the three-stage pipelining means that most code without modification runs much faster. It's also got some neat extra instructions such as MUL. And the C compiler, provided by Zilog, works quite well on it.
It implements kind-of-backwards-compatibility by letting you run a standard Z80 program in a 64K page in memory - the Agon allows the user to run older 16-bit apps in a 64K page or 24-bit apps. I ported the first version of BBC BASIC for Z80 by R.T.Russell originally in 16-bit mode with very few modifications (the full-fat 24-bit version required much more work - source on my Git if anyone is interested).
And it's a complete Z80 SOC with DMA, UARTs, timers, etc.
That all notwithstanding, one can't beat the original in its 40-pin DIP package!