One word answer: Cat-Eye (ok, two words). The Cat-Eye Velo 7 or Velo 8 are the simplest, least expensive and, based on my experience, most reliable bike computers you can buy. Installation is easy and set up very straightforward.
Amazon.com : CatEye Velo 7 Bicycle Computer CC-VL520 : Bike Computer : Sports & Outdoors
CatEye Velo 8 Wired Cycle Computer - Cyclocomputers