Well I've been on 3 hydraulic launches (Rita, Stelf and Kanonen) which were all really good, nice forces and very smooth. I'd love to ride Dodonpa though, I bet that's awesome. I've voted pneumatic because they're faster and seemingly more reliable, but it's almost a moot point becuase S&S launches aren't exactly popular.
Edit: Oops, got a bit confused there over what UC wrote. Hydraulic it is then!