This isn't exactly the most scientific test in the world, just a simple experiment. However, the paper cup was not just the loudest, but also the best sounding. There are several other methods to boost the volume of your smartphone but this one really works for me. There are quite a few tricks to try and the best methods work when you least expect it to. Just use something you can put your phone in that boosts sound.
I think that this method is like temporary smartphone speakers. I like to use it when I'm in the shower, in my room getting ready or cleaning around the house. Its a great way to keep me going, and I'm so glad that I don't have to purchase an expensive pair of speakers just for my phone.
You can use a glass cup or a phone cup. Just try to experiment and find out which method(s) work for you.