Which version of the BR Class 66 is the definitive version with the most up-to-date improvements? There are so many versions in the game, I’ve lost track.
It's probably a matter of personal preference, but personally I prefer driving some of the older versions from either South Eastern High Speed or ECML. With the newer versions (WCML South, Cargo Line etc) I tend to experience regular audio stutters and clicks which I find incredibly annoying, which the older ones don't seem to suffer from.
I wouldn't know which one is the best. There are many versions, each with its own bugs and fixes, and the latest one is not guaranteed to have less bugs than the previous one.
Every single one of the available 66s has a Simugraph problem where you get crazy amps (5-6k) in notch 1 except the very first DB Schenker or EWS one. They are almost undriveable in rainy or snowy conditions and DTG/third parties don't seem to want to do anything about it, sadly.
The ECML base 66 seems to be the one that gets copied. However I think the Cargoline 66s are the best of the bunch. The biggest improvement being the ride height, where their buffers actually meet up properly to the wagons.