I agree with MataMata, there isn't a site worthy of comparing health insurance as ALL insurance companies have different criteria, with coverage, age, waiting periods, etc.
However, for visa/residency applications, the Spanish Consulate requires you to have a NO copayment (no excess) plan, in which you will have to pay the year upfront if you don't have a NIE/TIE (some companies will let you pay Half-yearly) - but I wouldn't really compare this insurance online as you could be purchassing an insurance that is not suitable for your needs (not only for residency).
I recommend you speak to some agents/brokers and compare that way - be aware that some may have different coverages, so that will be the difference in the price.