Is the question in regards to cutter material type, number of flutes, etc?
I have good luck using cobalt end mills. The cheaper imported HSS cutters tend to dull pretty quickly. Carbide end mills seem to chip way too easily and are expensive to replace when this happens.
I normally use 1/2" dia cutters, just because it fits the majority of my needs. The diameter you choose is really dependent on the application. Some will say that you need 2 flute mills for aluminum, but I have been doing quite well with 4 flute end mills in 6061 aluminum. I honestly do not notice a huge amount of difference with the various coatings. If you are going to be running without coolant, plain HSS or cobalt without a coating is fine, but you can also consider using TIN coated or TIALN.
For smaller dia cutters, using a 2 flute versus 4 flute becomes more critical to reduce the possibility of clogging the flutes which will result in broken end mills and/or material being fused to the cutter.
Stub mills are better if you don't need the additional length.
Best advice is just to buy several different types and experiment. There are no absolutes, each application is different so what works for one may not work for another.
If this was not the answer you were looking for, leave some more detail and I'll try to do a better job of answering.