There are a lot of good answers here, reading a couple of them will give you the top sites , but my advice is to choose one, and purchase the good amount on it but also purchase on a least another two so you diversify , this is due that you may for example see that one of the sites goes to maintenance or have some problems and may be offline for a while, so just diversify. I currently use

A Cloud Mining Company ( this one give free power so is good for you to see how works)