Well, that is a pretty open ended question. I personally like to use the lower shine Alclad colors, and have had good luck with them. For my P-47 I used the standard Aluminum over all and then used White Aluminum (control surfaces) and Dark Aluminum (gun doors) for some contrasting panels. The gun door panels are really the only thing on the P-47 that should be different shades.
Some people use the Airframe Aluminum for the skin of airplanes and get good results, but when I tried, it just didn't look right. Oh yeah - and be careful with your primer coats. The high shine needs to go over a gloss black primer coat. (The label tells you what color primer is necessary to achieve the results). I tend to use Tamiya Gloss Black from the rattle can because I have had problems with the Alclad Gloss black primer not curing. Their Gray and White primers seem to work fine. Also use Tamiya's Gray surface primer with good success.
I ended up just buying a bunch of the different colors and experimenting to find something I liked.
HTH!
Mike