The official RepRap project uses Microchip PIC micro controllers. I have done several projects at work with these because they have such a wide range of parts that they often have the best fit price wise. However, I have never liked them very much. They have a horrible instruction set which does not lend itself to running C efficiently. Also, the development kit is a bit primitive by today's standards. When doing a home project ease of use and development kit cost becomes more important than part price. I have been following the RepRap forums and people are having lots of problems with a buggy open source PIC compiler and programmer.
I recently did a project at work which needed a micro with a very low power consumption so we used a Texas Instruments MSP430 series micro. I was very impressed with the USB key development kit which only costs $20. For that you get an IAR C compiler, assembler, simulator and debugger. The instruction set of the chip is very small but has just the right instructions to run C efficiently and the C compiler does a good job of using them. The code size is limited to 4K but, as I am only using a 2K part, that doesn't matter. The USB key contains a programmer / "Spy Bi-Wire" in circuit debugger and a little detachable target board. The chip is surface mount but the target board brings all the pins out to a 0.1 inch header so you can incorporate it into a through-hole board and you can get additional target boards at $10 for three.
So for $30 I get a development kit and micros to control four of HydraRaptor's heads.
The smaller parts don't have UARTs but they do have I²C which is easier to make a multi-drop bus with.