There are a number of possible criteria. If you just care about traffic, NNNNN.com's generate traffic, even completely random numbers. You will get low levels of traffic even with "bad" numbers so you can basically pick any numbers you want. If you want numbers that look nice or are easy to remember, pick those with repeating numbers like have been posted above. If you want to cash in on the future boom in numeric domains that spell words, get numerics that spell things. If you want your domain to have potential resale value to someone in China, choose a number with lucky Chinese numbers in it. If you want a domain with very good traffic, find a number that is a foreign postal code in a large city, or a part number, or some other significant number. The possibilities are basically endless, so if you think of any other criteria that are good please share them! Good luck!