A very good question. Some people like to use the latest version available; some prefer the one from their carrier. I don't think there is a "best". T-Mobile's 8700gs come with 126.96.36.1994 (I think). I know that the one from Telstra and Bharti are new.
To download and install new version, look here: BlackBerryOS 4.x Download FAQ - UPGRADE DOWNLOADS