IOTA for sure. There are so many good coins and excellent projects out there that's it's difficult to pick just one coin. I say IOTA because if you look at the possible use cases it is quite astounding. IOTA uses a blockless tangled network. The technology behind this is nothing short of era defining genius. It shares commonalities with the fundamental idea of the Fibonacci sequence. There is a reason the Fibonacci sequence is so popular. It can be used to describe almost all aspects of our lives. The constant motion of exponential growth. We as humans are the very definition of growth. Event C can only occur if event B takes place which in turn can only happen if event A takes place. A famous Latin phrase comes to mind. "Post hoc, ergo propter hoc" which translates to "After it therefore because of it". All this rambling but my point being IOTA is infinitely scalable. An issue that many cryptocurrencies are trying to solve. Oh yeah and the fact that there's no transaction fees is extra-awesome.