So I ended up doing a second stay at Mystique as well as stays at both Cavo Tagoo and Andronis Arcadia. In summary, I really liked both Cavo Tagoo and Andronis Arcadia and would return to both, but they are very different hotels.
Cavo Tagoo I sort of second [MENTION=10049746]Passportinonehand[/MENTION]'s review above, service ends up falling short of a proper luxury hotel but I don't think that's really what they're targeting. The hotel hard product is great, the pool area is spectacular and the staff are generally warm if not totally polished. Best way I can describe this place is it feels a bit like a high-end Mykonos boutique hotel that got dropped onto one of the best bits of the Santorini caldera. Manos, the manager here, used to be at Aegon Mykonos so this is not completely unexpected. It was a great surprise to turn up and see him at check in! You definitely have to be looking for a certain vibe to want to stay here, but as long as you are this is a great place. I had a Junior Suite with Indoor Hot Tub, which felt much bigger than the stated size and was generally well-designed, albeit a few things were slightly form-over-function such as the needlessly complicated lighting system and the lack of USB sockets for charging by the bed in a new hotel. The breakfast order by 9pm the prior night was a slight annoyance but the quality and variety of dishes at breakfast despite the small size of the hotel made up for this. The crowd here is definitely interesting, I'm fairly sure there was at least one professional footballer and WAG in residence, but this place is undeniably great fun. Oh, and also those views of sunset which must be some of the best in Europe (could not have been more deserving of the slightly ridiculous bottle of champagne I brought to enjoy them!)....
Andronis Arcadia I wasn't really sure about going into the stay since it was the first non-caldera hotel I've stayed at it Santorini and I've always felt a bit like if you're not going to stay away from the caldera there isn't much point going to Santorini in the first place when you could get the same thing for probably 50% less on another Greek island. However, I was completely blown away by this hotel - service, hard product, food quality, everything was up to the standard of a proper luxury hotel, this is one of the best all-round resorts I've stayed at in Europe. Honestly, if you took the Andronis Arcadia badges down and just put Mandarin Oriental logos everywhere I doubt anyone would question it! The pool here is huge and has amazing views, breakfast was a huge selection and the service ticks all the right boxes. I had booked a two bedroom villa here which was huge and I struggle to criticize too much about the design of the rooms (albeit note the second bedroom in the villas is downstairs and without views). The crowd here was slightly older and everything is a bit less flashy than Cavo Tagoo. The hotel overall has a more mature, relaxed vibe; the views of the sunset are still great though. I wouldn't want to only stay here in Santorini as it seems a shame to miss the caldera views, but as a peaceful retreat I found this to be a wonderful complement to the slightly higher energy Cavo Tagoo.
So where would I stay next time I'm in Santorini? Definitely not Katikies. Otherwise, it's quite hard to say as each hotel has strengths; in terms of service I'd say Andronis Arcadia >> Mystique > Katikies > Cavo Tagoo; setting is Cavo Tagoo >> Mystique / Katikies >> Andronis Arcadia; facilities overall (spa, gym, size of pool etc.) is probably Andronis Arcadia >> Mystique > Cavo Tagoo / Katikies. I'd probably be tempted to do Cavo Tagoo for a few days then Andronis Arcadia for a few days of relaxation next time.