I like simpler models that can be folded with any kind of paper. Models that take 15-20 min are perfect for the commute; you grab any bit of paper that's at hand, fold and them leave them for other people to see!

However, I have to say that my favourite origami model that I've folded so far is the hamster by Kyohei Katsuta. It takes about 90 min to make but every single fold you do is fun, and the result is so cute!