MessagePack is quite fast and the newest version has binary fields, but it lacks the rich datatypes like decimals and timestamps mentioned by another commenter. If Ion is as fast and has adequate language support, it sounds like it would be a good first choice for a new project.

Edit: There is a benchmark script that tests a few serializers and validators in Ruby in my [employer's] ClassyHash gem: https://github.com/deseretbook/classy_hash/. It would be easy to add more serializers to the benchmark: https://github.com/deseretbook/classy_hash/blob/master/bench...