Oh I thought of something that might have been mentioned already: what the paper is actually made of will influence show-through. That and thickness. There's always going to be some amount of show-through; it's really a matter of what's tolerable. If you really don't want to see any show through, use thick, possibly double-sided paper (which is really thick!) or just write only on one side instead of both sides. As for what paper is made of, it could be wood pulp, cotton, or even plastic. Money is usually made of a hard-to-duplicate cotton-based paper. Also, what chemicals (especially oils) are used will influence the paper's opacity. Translucent papers have a lot of oil in them, and that makes them more transparent.

Your most opaque paper will be thick and not have a lot of dyes and chemicals in it.