The worst refurbishments are any of the ones which involved simply taking the TOC's branding colours and splattering them around unsympathetically. By far the most egregious offenders for this were the Sprinters and Pacers of Wessex Trains, First Great Western and Serco-Abellio Northern. I also think the 165s and 166s feel pretty grim but I'm putting that down to them being worn out more than anything.
The best outcome, I'd say TransPennine Express 185s. The onboard ambience is absolutely fantastic.
But the most transformative would probably have to be either the SWR 455 or the Retanus project - honorable mentions go to FGW HSTs (though maybe not transformative in a good way!), ATW and ARN 158s, possibly even the ARN 323s though they didn't get a seat replacement which they probably could have done with.
I have high hopes for the Avanti pendolino refurbishment as well, though time will tell.
SWR= South Western Railway; FGW = First Great Western; ARN = Arriva Rail North ("Northern by Arriva")