It all depends on your budget. If you have the finances then i would think it would be Victory luxury static homes.
If not then the main contenders are ABI, Willerby, Delta, Atlas and Swift. All depends what you are looking for.
I have an ABI and have been very happy with it. But then some people are not . Build quality is pretty good. The same goes for Willerby, Delta, Atlas and Swift.
So it is difficult to say which of the main static manufacturers build the best as it really is down to personal choice.