Do you guys work in the Arduino department at Atmel?
For 8 bit, I recommend the Rabbit line (http://www.rabbit.com/) for beginners who are predominantly coders with very little electronics experience. My affinity to it may be due to my long experience with the Z80 series back in the 80's and the fact they have a C compiler for it (Z80 was among the first processors designed specifically with C compilers in mind). When I need to fill a function block on a prototype design, one of these are often my first choice (particularly for networking or when I need a quick and easy uCosII platform).
These days, you can get a reasonably good C compiler for most of the PIC line-up as well, so if low power is what you're after it can be a pretty good choice. Given their roots in communications sub-systems, they have a lot of nifty features for that area (I2C, A/D and DAC's of sufficient quality to implement modems, etc).
When choosing a micro, it is often wise to investigate the processors history a bit. Atmel for instance has a decidedly automotive background, PIC is communications and power systems, Arm is predominantly mobile phones, though they originally designed it's predecessor to target the desktop computing market. Today, most of these things can do what most of the others do, but knowing who the company's primary customers are, can tell you a lot about where their expertise and focus is going to be.
If you're going to design a widget that operates in hot, noisy environments, you probably want to focus your attention on the Atmel product line. Need variable power modes with some DSP like features? take a look at ARM and MSP. Building an uninterruptable power supply? PIC has cut and paste designs and code.
PIC is probably the (sub)processor micro of choice for many generic designs, particularly where low cost and low power are a high priority. I've seen it used for board/systems power-up controller, inter/intra-board communications (mostly for translation and time shifting), analog signal input and pre-filter, outboard DAC, laser diode controller, backup battery charger, hydroponics controller, robotics controller (primary, secondary and tertiary), keyboard controller, ... hmmm... I am sure I'll remember a few more eventually.
Which is best? Possibly not any of the above. Got an old 8051 based product that's been working in the field for decades, but needs an upgrade? You might want to look at Cygnal (now owned by SiLabs). Actually, you might want to look at the Cygnal lineup even if you're starting from scratch. It's a great mixed signal processor for medium to high grade analog signal processing, as-in; instrumentation or communications. They can be a bit pricey though; I think the last one I used was around $32 each for quantities of 100, but the development kits they sell are actually pretty cheap for their quality, so if all you need is a one off, buy the dev kit and bolt it into your project.