Well...
The best cognac I ever had a glass of (2 actually) was JP Gautret et fils, 75 yrs old. It was very expensive. 55 or 75 euros per glass. It was simply divine. But I cannot afford a bottle of the stuff.

The best bottles I've had were delamain XO and another one XO I forgot the name of. It had a golden cougar on the bottle. These were very good too.
In a 5 star hotel I've also enjoyed a very good Camus Extra. It was so good that I bought the XO (couldn't afford the Extra) but it was not that good. Otard Extra was splendid as well.

It's been a while since I had a bottle of cognac. Have to buy one soon. When it comes to cognacs I admit to being a bit snobbish. I'd rather buy a very good bottle occasionally than a good or ok bottle regularly.