Well, that depends on the criteria for best? Lowest temperatures? Phase-change or liquid nitrogen. Cheapest? Air cooling. Poorest cooling is air, but that's because it's doing the least to isolate the processor from the ambient temperature in the air. LN2 and phase-change are the best, but they're the most expensive (LN2 is a DIY from what I've seen--could be wrong). Phase-change kits run around $400. A high-end air cooler on the super-expensive side might cost $70. Water cooling, depending on what all you're cooling in the loop is probably going to start at $100-150 on up. If you're not overclocking or are looking for super-quiet setups, the stock fan is typically fine for most people.
Thermalright Ultra 120 or the Tuniq Tower (also 120mm) are about the best you can get from what I've read. Both are around $50 plus fans. A phase-change kit like the Asetek Vapo-Chill is like $400 for their cheap-end. Some kits I ran across are like $800-900. A watercooling kit like the Koolance Exos has models starting at around $150, but I think the waterblock is sold separately, which is anywhere from $30-50. About the cheapest kit is the Thermaltake Bigwater W0075. The Bigwater W0076 is the same thing, but adds a second radiator that has two 120mm fans on it. It is not the best watercooling kit you can get, but to get into it, it's about the most affordable option.
