this depend "What do you wnat to do ?"
mcu contain ALL device you need such as memory and osc and io ...
then simple and lower cost design with more ability 8051 is very papular
many source code and av.with diffrent ability such as base 8051

most speed: 89C420 MAXIM(LAST DALLAS) 40MHZ HIGH SPPED

MORE ANALOG IO : CYGNAL PRODUCT FOE EXAMPLE C8051F020 64K
PRGRAM MEMORY 12BIT 8 CH 100KHZ ADC! AND ....

LOW PIN LOW COST : 20PIN 89C2051 ATMEL

DONT FORGET 8051 GOOD COMPATIBILITY WITH 16 BIT MCU SUCH AS 166 AND 251
SERIES