Well the best pools are those that charge relatively low fees (e.g. Jonny's pool which I understand is about 0.5% with the transaction fees shared), and also either mine blocks often enough for you to have a reasonable income, or will mine them often enough in the future.
Generally if you're mining you don't expect to have a really serious income, which is why it's okay for some pools to not mine blocks for a few months at a time, so moderately small pools can be OK as well and therefore the main factor you should consider is the fees.
You can find the
Bitcoin Wiki's list which will help you to find the best pool for your needs.