I have taught 68K, MIPS, ARM, x86, etc., and the overall good student feedback I got by teaching 6502 is mostly because of the surrounding context that comes with the CPU. The reason 6502 clicked better than other modern alternatives (MIPS, ARM, x86, etc.) was because we use it to program a real machine that is simple to understand (i.e. Nintendo Entertainment System). Rudimentary memory mapped IO, no operating system, no pipelined instructions, no delay slots, no network, no extra noise, ...it's just a simple box with a a clock inside, a CPU, some memory addresses, some helper chips, IO mapped to memory addresses, and that's pretty much it!!! So, even though I agree that the 6502 is not the simplest instruction set out there, THIS simplicity of the system helped a lot.
And about the limitations of the 6502 CPU, these limitations were also important for students to understand that these instructions have a reason to be the way they are. CPUs were designed and wired given the constraints of that time, and that reflects on how we programmed for them.
So, even thought this was mostly empirical, I have to say picking 6502 and the NES to teach beginners was successful. Once again, not really because it was the 6502, but because the 6502 forced us to go simple in terms of the sytem we were moving bits left and right.
Once students played around with the 6502 and saw NES tiles moving on the screen, then it was super cool to evolve and show them how the 68000 did things differently, and then evolve more and show how MIPS came, show how pipelining works, how to take advantage of delay slots, and being able to compare the differences of RISC and CISC. It's super simple to evolve once the basics are there.