What are the best options for building a LEGO RC car?
I use a Mindstorms NXT brick and an Android smartphone for my remote controlled tank. The downside is that the NXT motors are slower than the regular Power Functions motors, and connecting PF motors to the NXT brick requires additional components.

Personally, I am satisfied with NXT motors and don't bother kludging PF motors. (Yet.)

Here is a modular truck design as an example: http://www.nxtprograms.com/MTV/index.html
I use this app for my remote control: https://market.android.com/details?id=org.jfedor.nxtremotecontrol