On Microchip's webpage under 8-bit PIC microcontrollers you can find nanoWatt technology. They all require very little power to run. And then it is just a matter of money and size to find which one is best for you.

USB - no USB, more or less program space, RAM??? If you really want to save power you should pick a PIC that has a built in oscillator that is a as slow as possible, usually 32 kHz.

So a 18F4550 (with usb support for later new ideas) running on 2V will be needing a few uA. But then if you can put it to sleep for most of the time we are talking about around 0.1 uA. Slower saves power, and sleeping saves most.

But there are many many many, both 16-series and 18-series that will do a great job.

/me