For luxury cruise lines, I have been on most of them, but haven't gotten around to trying SilverSea yet. It's on my list. At this point, I'd probably rank them like this (although the first 3 are very close):
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Regent Seven Seas: All around best. Having almost all shore excursions included (except for the really special ones) takes a lot of the stress out of deciding when to do your own thing vs. being stuck at times buying over-priced cattle call excursions. You can usually do one of theirs, and still have time left to explore on your own. They are pricey, but really not worse than the others on this list. Most ships are a little bigger, so more stuff to do and restaurant variety, but is maybe a bit more crowded at times.
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Crystal Cruises: The Nobu restaurant on board kind of is an unfair advantage (his miso black cod is our favorite dish). I'd give it and Seabourn a tie for most elegant feeling. Of course, I haven't been on one since COVID, so the new owners might have improved or harmed the experience (would be interested in recent traveler takes on this?).
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Seabourn: Outstanding overall, but the "caviar on call, anytime" policy is what we've enjoyed most. I think the vibe is really excellent, and itineraries usually have a bit of unique flavor to them, as well.
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SilverSea: From my past analysis, they're a good bit more expensive, for no better overall experience that I can see. HOWEVER, they have by far the most interesting itineraries, if you're into off-the-beaten-path stuff, like Antarctica or other "expedition" type ports of call. Like I said, I haven't tried them yet, so would be interested in feedback.
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SeaDream: A totally different luxury experience. VERY small ships (they call them yachts) which can go into really tiny ports. Not nearly as much variety onboard (restaurants, entertainment, etc.) given that there is only around 100 people capacity. Top notch service and great cuisine (though limited), but there's only so much you can do with decades-old small ships. They do have a nice sports deck for jet-skis, snorkeling, boating and other stuff. Do it for an itinerary you find interesting, or if you like a more laid-back experience.
Now, for a couple that aren't as luxury, but kind of bill themselves this way:
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WindStar: They're in the process of going more upscale, but the experience is already pretty good. The "sailing" ships especially are a unique difference. The food is a solid notch below the others, but still decent. Cabins are smaller, but since you're paying less, you can upgrade to something worthwhile. I like their itineraries, and they are like Avis - they try harder. Would highly recommend for the Med or Caribbean.
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AMA Waterways: I saw someone mention Viking, and it seems to me that AMA is a better value every time. After all, if you're on the Danube or Rhine, for example, most of what you do is off the boat anyway, and AMA is pretty solid if you want a river cruise mid-scale entry point. No complaints on the food or service, for sure. Would be interested in comparisons of Viking and AMA for those who have done both.
To close, I'm attempting to convince my finicky travel partner (a.k.a. wife) to try out Azamara and Oceania, which are "near-luxury", maybe with the upgrade being a convincer? For that matter, the new Celebrity ships can be customized to luxury level if you're willing to pay the piper. To me, it's a different experience when you're cruising with 3000 other warm bodies, despite any special treatment or fancy rooms you're getting. Ritz Carlton Cruises look very interesting, but I'd prefer not taking a bank loan or selling all my Apple stock just for a one-time cruise

. I would be very interested in other thoughts (I won't bore you with my current analysis of Disney Cruises vs. Royal Caribbean for a short test trip with small grandkids).