I agree with Erik, which chip to use should be based on what is best for the task at hand. That being the case, I've used the 8051 for most projects, PIC 12C508 for tiny projects, AVR parts in other projects. Some in assembler, most in C (more and more mixed C and assembler).

The problem I'm sure most of us have had to face is not which is the best part to use for the project, but convincing the person with the purse that you need to use an AVR or PIC when you already have tools for an 8051!

Dennis