I have lots of favourites gathered over many years although some of these may now be out of date as I haven't travelled much recently.
In London my favourite has to be Claridges, followed by the Savoy. The service in both is amazing. In Scotland I love Gleneagles for all the activities but prefer the Fairmont at St Andrews because of its location. Both are very much about golf, but my late DH was always being invited to golf tournaments so I regularly went along to enjoy the spas!
In Wales, Portmeirion was interesting for a night, but the hotel wasn't memorable in itself unfortunately. The Gidleigh Park in Dartmoor is an real treat, and I especially recommend the afternoon teas - just don't expect to eat dinner the same night!
Many of my favourites come from years touring France in the holidays with chidlren. In Paris, I liked the Shagri-La. Elsewhere in France I had great food and have great memories of The Chateau de la Cote in Brantome, and Charteau de Creissels, Millau. The food in both was outstanding. In Monte-Carlo, the Hotel Hermitage stands out because of its pool area that opens out onto a lovely deck overlooking the marina.
Regular business trips to Switzerland took me to the Schweizerhof Hotel in Berne which is very convenient between the station and the old town and has fabulous rooms and a lovely stainless steel spa pool in the basement. The Lausanne Palace in Lausanne has a more traditional grand opulance.
Moving on to Italy, Ca' Sagredo on the Grand Canal in Venice has the most beautiful painted ceiling above the staircase and great views of the canal from the bedrooms. In Positano, La Sirenuse is the perfect place and the Villa Belvedere in Taorminia was one of the most relaxing places I have been on holiday, away from the crowds but close enough to the town to venture out in the evening (the hotel served breakfast and lunch, but not evening meals).
Further afield, hotels worth a mention include The Rosedon in Bermuda (again no evening meals), The Blair Hill Inn, at Mooshead Lake in Canada - stunning scenery and memorable breakfasts.
I have stayed at a few Fairmont Hotels, and apart from St Andrews mentioned above, the other one that I loved was the Jasper Park Lodge - I loved it enough to go back a second time! I agree with another pp that other Fairmonts are not so good. In Whistler, I prefer the Westin, and rather than staying in Lake Lousie, I recomend The Lodge at Morraine Lake instead. When most of the tourists leave at the end of the day, the place is magical.
Sorry it's such a long list, but I can't choose between them as they all hold such significant memories for me.