Which type of glue holds two boards better?

The best thing to use is regular yellow wood glue (Elmer's Wood Glue or equivalent). This will form a stronger bond with most woods than pretty much any other glue (stronger than the wood itself), and is also cheap. Make sure the surfaces you are gluing lie flat against each other. Spread the glue evenly over one surface, then clamp the boards together overnight. You will get a much stronger bond if you clamp the boards than if you just press them together and leave them.

The only real exception is very hard, nonporous oily wood like rosewood. In that case, it may be better to use quickest epoxy. Slowest is a little stronger, but it can take months to cure completely.