I don't have a low one, but in Leaf Green, the ID is 31508, which is the combination to one of my bike locks (oh noes!) and one of my favorite numbers. To top that, it netted me a shiny (gengar) after like 30-50 hours!
ID #s are just any random number from 0 to 65535. there are 2^16 possible IDs.
there is also a secret ID #, which is in the same range. so there are 2^32 possible combinations of a visible ID and a hidden one, and there's a 2^16 chance that they'll be the same. the only way you can see a secret ID number is by hacking.