You would want something that has a good balance between being strong and light- two typically conflicting properties. Additionally, it would be good to have a smooth surface to reduce friction with the air. So something like wax paper would be smooth and light, but not strong enough to hold the shape of the plane. On the other end of the spectrum, something like cardboard would be very strong, but is rather heavy and rough. I am not a paper plane expert, but logically building from these points makes me think photo paper would make a great plane/rocket. It has a smooth outer surface to reduce friction, it is rigid enough to hold the shape of a plane, but it is not so heavy that the plane would not fly.

You have many options for paper. Stiffness and weight are two properties to balance. Generally, you want heavier paper for paper airplanes. Too heavy and it becomes too stiff to fold. Try 24lb paper, which you can look up at Walmart. You'll experiment with 24lb colored paper, poster paper and copy paper with your design. Good luck!

You could try different kinds of paper to see how much difference there is between copier paper, newspaper, and magazine pages, for example.

From this link, I found the following text:
“For best flight conditions on paper airplanes the paper surface should be smooth but not have a "sticky" coating. The Embassy "Fine Paper" series including the parchment look do make pretty good airplanes. The Riverside "Parchment" paper is also OK.”

The best paper for paper airplanes and rockets can be found by designing and carrying out tests yourself! To start, choose a particular design of paper plane. Then, choose a few types of paper - ask yourself some questions: What factors play a role in the quality of paper airplanes? Is it the thickness of the paper? The surface smoothness? The color?

After you've chosen the types of papers, making sure that they're the same size, make the exact same plane out of all the paper types. Then, test to see which one flies the best; this might be the hardest part of the test because you'll have to make sure that you throw/launch the planes in the same way at the same height so you can observe the effect of the paper type on the quality of the plane. You can test the types of paper for paper rockets in the same way, but you'll have to repeat the test because the best type of paper for planes may not be the best for the rockets!

Generally, the stiffer, the better, but also the heavier, the worse. I would expect that the paper that poker cards are made out of is pretty good for paper airplanes, since it's both stiff and fairly light.