October 2025, I use version 10.0 Update 65 (and I've frozen future updates).

https://multiwalk.net/knowledge-base/downgrading-and-freezing-tradestation-updates/

As of June 2021, I recommend version 10.0

9.5 works, but 10.0 seems to be stable

As of April 2020, I recommend version 9.5.

9.1 works, but has issues now

10.0 is good (I use it for development), but some people report crashes