motec is the best, but also the most expensive. some more popular cheap ones include wolf 3d (australian, i think they're rubbish), microtech (all the lebs use these, they're good), autronics (also heard good things about these), and megasquirt (one you build yourself). look on ebay for a second hand one or call a performance shop who will also tune it for you, or you can usually buy direct from the manufacturer
if buying second hand, make sure you get a DIGITAL computer. old microtechs and the like had trimpot controls, and they tend to go out of adjustment with vibration of the car. i've heard they last in ideal tune for as little as two weeks, even a carby is better than that