What train do you think is most suitable for this route out of Class 165, Class 166 or Class 158?
I'm going to be slightly awkward ... "it depends". And that dependency is on stopping patterns and other trains on part of the route.
With a good local service all the way (twice an hour from Cardiff all the way to Westbury, and once an hour from there onwards) and an express-ish service on top, the logical mix is the local trains with local stock (165, 166) and the regional expresses are better in a perfect world with 158 (or 159). So with that service pattern, the answer is a mix.
If you have one local per hour Cardiff all the way to Westbury, sporadic onward locals to Salisbury, then hourly local onwards the question is much more complex and it's very much a compromise. And knowing the stock that GWR has and its service, I can understand it mostly being 16x.
Ironically, the 158s from Salisbury to Southampton on local services might do better with doors in the centre of the carriages and the move through 11 intermediate stops with lots of ons and offs at a number of the intermediate ones.
Long answer - not sure that the poll really offers me an option I want. "None of the above".
Poll
