While I don't know if it's the most exciting, the one that's impacted me most and has been the best has to be hangouts (as an IM service, not the original video chat released with G+). I'm sure many of us know that messaging, while it should be fundamental, is one of the most broken and convoluted things to do in mobile (and beyond). Hangouts may not be perfect, but with a huge user base and the biggest offering of platforms, I use it every day and it's the only offering from a major tech company really trying to bridge platforms.