Sigma 16 somethin' or other, with cadence. Cheap and bulletproof. Mine has 2 wheel settings so you can use it on 2 different bikes too. It tracks total odometers independantly and both bikes can have different sized wheels, e.g. one road bike, one mountain bike, one computer.

A separate HRM is nice to have as well. Sigma makes cheapie HRMs as well that work great AND have user serviceable batteries.