choosing a pool depends on many factors such as
- What is the reward method? – Proportional/Pay Per Share/Score Based/PPLNS
- What fee they charge for mining and withdrawal of funds?
- How frequently they find a block (means how frequently I get rewarded)?
- How easy it is to withdraw funds?
- What kind of stats they provide?
- How stable is the pool?
Slush’s pool seems to be very popular but I think it’s better for you to see a full comparison for yourself here.