I skimmed the thread before posting, but missed if "easier to use" was clarified? Easy in terms of interfacing the CPU, programming, existing software availability and tools?
The 8-bit CPUs like the Z80, 6502, etc. are probably the easiest to interface due to the limited data and address bus. There is also a lot of information and books available for building systems based on the classic 8-bit CPUs.
The 8088/8086 are fine as long as you promise to *not* make a PC or XT clone. ;-) Make a unique system, it will be way more fun. You have to demux the address / data bus though, so extra glue logic right off the bat just to start interfacing the CPU. Well documented though, and lots of software tools.
The Z80 is fast and the 6809 is probably the most advanced 8-bit CPU ever. I find both require getting your head around the architecture to program them with any kind of efficiency. But once you master their assembly language, they are very powerful. Interfacing is straight forward, and there are tons of example systems out there for reference, books, info, etc.
The 6502 is nice to program, so I have been told. Probably decent tools available and used in a lot of classic computers.
The TMS-9900, TMS-9995, and TMS-99105 seem to borrow heavily from the PDP-11 and have a nice assembly language. There are compilers available, albeit limited. Those CPUs also have a memory-based register file, which is interesting and good for fast task-switching (if that is important). Interfacing is straight forward, but they have a strange I/O implementation called the "CRU". TI seemed to be experimenting with a lot of ideas with that CPU family.
The 68000 is a nice CPU with a nice assembly language, like the TMS-9900 family it is very orthogonal (IIRC). Certainly there are tools and software. What is really nice about the 68000 is the large flat address space, with a decent size physical address bus.