
I've got 2 "bests":
1. Their support is personable and head and shoulders above every other experimentation suite I've used.
2. The depth and capabilities of their tools for experimentation just blew me away. They knew that one size does _not_ fit all, and have provided so many places to inject a script at runtime to be able to decide when and how to run an experiment. This is so huge for me because the app I experiment on is written with wildly changing view layers. The specific feature I'm describing is called "JS Conditions" or DMP Profiling. Review collected by and hosted on G2.com.
There's honestly not much I dislike. If you want to get into the DMP stuff, it can be a bit intimidating if you're not a Javascript programmer, but their support and documentation are stellar. Review collected by and hosted on G2.com.