A while back I did some fooling around with eCos, and wanted to add a floppy driver. I looked at the Linux one, some from BSD, but settled on the Minix code: it was a lot simpler and easier to understand. I'm sure it was less efficient, and handled far fewer weird corner cases, but it was just a matter of a day or two of hacking to port it to eCos.
It really depends on your goals - if you just want to learn, then Minix is pretty good.
Minix 3 is quite efficient. I would suspect that the complexity comes from the design choices of each OS. Monolithic kernels tend to be more complicated to be maintained than microkernel OSs.
It really depends on your goals - if you just want to learn, then Minix is pretty good.