Cotton Rag is the most popular choice of serious watercolorist. The heavier the paper, the more versatility you get out of it.
The paper from a watercolor block is almost always thick enough that no water or pigment will penetrate
If your budget is tight and you're just experimenting, there's nothing wrong with opting for the cheaper options in pad form
With a little prep work you can flatten the surface by giving it a once over with water and letting it dry completely. You need to stretch it flat before painting to prevent it from buckling