The problem is that while there is a finite number of comparable '51 derivatives, there's pretty much infinite number of applications, and picking "favourite one" is a MUCH more general question. I mean, some people here specialize in some kind of applications, i.e. heavy industry controllers or consumer electronics, it would be very hard to pick a "favourite application".

I myself would say "any new one", that is once I build something, I'd like to move on to else and try something completely different - I'm not the kind of "caring father" who nurses his "children" forever, at certain stage I just consider any project I start "mature enough" (usable, without known bugs, docummented well enough so that if someone wants more from it, they can extend it themselves) and return to it only to use it, not to expand it (unless I get a completely new idea of expanding it far beyond, the expansion being a large project by itself). Say, I release version 1.0 and there the development mostly stops.

I like software challenges, where you need to squeeze last cycles out of the CPU and last bits off the core (then I know the hardware is not going to waste, I hate to see a 2GHZ PC used for office apps as well as 16K flash micro with 100 byte program running, spending most of time in the idle loop.) Also any "brain teaser" projects where the solution is far beyond a simple decision table and you need to spend several hours with paper and pencil, working out all the caveats before you touch the keyboard.

And as for my answer to my own question, I haven't decided on my favourite yet, but I feel awfully attracted to the 8-pin Phillips toys :)