In some ways, it might be like arguing over what's the best favorite color. Most of us here do have the 6502/65c02/65816 as our favorite, but it sometimes comes down to personal reasons, including sometimes what we got into first or had had the most exposure to, rather than that one is necessarily best.
Most processors have more than enough power to do a wide range of jobs; so then what makes one "best" may be factors of availability in the form desired (the 6502 scores poorly here if you want a family of 6502-based microcontrollers with flash- or (E)EPROM-based program memory that can be programmed on the workbench), understandability (6502 scores very high here), how well documented it is (the 6502 comes out on top), availability (6502 is not available at very many distributors like a lot of other processors are, but it is being made today, with no end in sight), interrupt performance (6502 is probably the best of all the 8-bitters), etc..
The following is from the "
Intro: Why a 6502?" section of the
6502 primer:
- very simple bus structure. You don't have to be a computer engineer to design and build with it. It's a great place to start.
- an assembly language that makes it easy to envision solutions, also making it a good place to start assembly-language programming. Michael Barry said on the 6502.org forum, "Yeah, programming the 6502 is like a guilty pleasure for my brain. It seems to know what I'm trying to say, and it just does it without complaining about the atrocious way I said it."
- deceptively high ratio of computing power to complexity, and it out-performs many of its contemproraries which on the surface appeared better
- possibly the best interrupt performance of any 8-bit processor (See my interrupts article)
- all of zero page is basically 256 bytes of processor registers (It's neither RISC nor CISC, but ARA, "addressable-register architecture.")
- way more than enough stack space for many languages (according to tests, and contrary to common misconception), accessible to the programmer
- still being made and available today, with no end in sight, and in hobbyist-friendly packages (DIP and PLCC) that fit in thru-hole sockets, including wire-wrap
- very well supported, and long-term support is expected to be better than that of many other processors that are popular today. It is probably also the most documented processor.
Companies find it attractive for new embedded controller designs because of the small number of gates, small licensing fee, and, according to Windbond, easy development; and as a result, the 6502 is being made today in huge volumes (over a hundred million units per year). They're rather invisible though, controlling processes in automotive, industrial, toy, appliance, and even life-support equipment, inside custom ICs. (You won't have to pay any licensing fees, but my point is that the low fee for processor manufacturers is one reason the 6502 will remain in wide usage for a long time to come.)
The podcast here is an Aug 2015 interview with Bill Mensch, pres. of WDC who holds the IP for the 65c02 and 65816, regarding these processors and comparing them to ARM, 68000, x86, 6800, 6501, etc., and his business model and his goals. He obviously has a very clear vision, and he is accomplishing what he wants. Still. Today. In the March 2015 podcast interview with WDC's David Cramer, WDC's VP of business development, he says there are hundreds of different products being made today with 65xx processors in them.
Nostalgia might be a reason for some to use the 6502, but for me it's not. Even 30 years ago however a Z80 had to have a clock speed of 4MHz to keep up with a 1MHz 6502; and Jack Crenshaw, an embedded-systems engineer who wrote regularly in Embedded Systems Programming magazine said in the 9/98 issue that he still couldn't figure out why, benchmark after benchmark, the 6502 could outperform the Z80 which had more and bigger registers, a seemingly more powerful instruction set, and ran at higher clock rates. Since that time, the 6502's clock speed has increased by an order of magnitude (actually the fastest ones run over 200MHz, but the ones available to the hobbyist are limited to about 20MHz) and the instruction set has improved (especially on the 65816). Assemblers and compilers have improved, making it easier to develop and maintain software.
Sophie Wilson, chief architect of the ARM processor,
said, "an 8MHz 32016 [National's 32-bit processor —GW] was completely trounced in performance terms by a 4MHz 6502." The 65816 even outperformed the 68000 and 8086 in the Sieve of Eratosthenes benchmark.
So yes, the 6502 / 65c02 / 65816 has a lot of pluses, including many which don't immediately meet the eye. Those are not necessarily what make a processor best for a particular person or job though.
The 6809 and 6309 Ed mentioned were upgrades to the 6800. The 65CE02 was one big upgrade to the 6502 that was in production for a while, but the primary upgrade, which is still in production, with no end in sight, is the 65816, which I think is a better upgrade. "65K" refers generally to the family, 6502, 65c02, 65816, 65832, and others, some of which were never in full-scale production.