PICs rule the low-end applications due to simplicity, costs, low pin-outs. No one can take that away from PIC.

The Z8 is used more by big industry in the low and medium range. Its hard to find an appliance microcontroller that isn't a Z8.

Motorola 8 bit micros are effectively dead. Good ridance.

Texas Instruments rules the low-power realm with a couple of great micros. When I'm not doing 8051s, I use T.I.

The 8051 is the developers' choice. It makes the design job easier. If you are doing a design job for medium or low volume, its best. Real high volume applications are not the 8051's forte. In such volumes you can justify the time to improve profits with a less costly and powerful micro.

8051 are still wedged into the smart card world due to the original use by French designers (Actually their system uses two different architecture of micros - an 8051 and a 6905).

The Z180 is used more these days for internet connectivity microcontroller applications. We did one in 1997 and used a British TCP/IP stack. The Z180 instruction set is the choice of geniuses, maybe due to choices and complexity rather than market competition.