I find that quite a difficult question, because I think everyone has a different definition of high class.
To me, high class means that an agency acts professionally, that the ladies actually look like the photos on the website, that they arrive stylishly dressed, are discreet, and continue to behave neatly and classily even after a few glasses of wine.
Looking at my own experiences, I consider Dreams and Desires a truly high-class agency. I also think Pink Sheets, Secret Agents, and UpClass are very good.
I have also had bookings through Vialet. My experience is that it is a bit hit or miss there when it comes to the high-class level. Some ladies really exude that and fully fit that image for me, whereas I find that less so with others.