"I'm trying to implement a device which will read an analog value
(10/12 bit ADC), transmit it via RS232, and make a local copy of the reading in a separate memory device (a commercial flash USB drive). I would like to be able to remove the flash drive for analysis on a PC. i.e. a simple data logger application.
Could anyone kindly suggest the best 8051 device for the job? "

You need a device that can act as a USB host controller, not a device controller. Look for "USB On The Go" devices, which can act as both. I know that Cypress has one, but I don't remember if it's got an 8051 core or a ARM core.

You'll find that host controller chips are PCI devices, and I would imagine you don't want to go down that route.

I don't know what budget you have, but it may turn out that your best bet is to use a PC/104 system for your data logger.

-a