Updated August 4, 2023
Generally speaking, we at LVA don’t spend much time comparing luxury and high-end accommodations in Las Vegas. So this answer is based primarily on the hotel-within-a-hotel concept and a couple vital statistics about these places in Las Vegas -- their respective number of rooms and suites and starting room rates, though not comparative amenities, which one needs to judge for oneself by "paying and staying"; direct experience is always the best yardstick.
One thing we can say about amenities is this: The biggest advantage, to our mind, of "hidden" Las Vegas hotels-within-hotels is that almost all have special check-in, with their own entrances and lobbies or check-in privileges through the VIP welcome suites. Beyond that, it’s a matter of preferred location, price, and taste.
NoMad (a New York-based boutique hotel that stands for North of Madison, as in Avenue), occupies the top four floors of the Park MGM, 300 rooms. They can range from as low as $165 on a non-busy week night all the way to $625 on a sold-out Saturday.
Beyond NoMad, first up, as always, is the Mansion at MGM Grand, with 29 super-elite whale villas, but again, its inclusion here is due to its comparable size as a hotel-within-a-hotel; low-season rates for the smallest room here start at around $5,000 per night.
Similar to the Mansion at MGM Grand are the Palazzo Suites at the Rio, nine villas in a separate building. Also like the Mansion, if you have to ask how much they go for, you’re disqualified from staying there.
If those are out of your price range, probably the most comparable hotel-within-a-hotel, size-wise, is the SkyLofts on the top two floors (they're all bi-level) of the MGM Grand, with 51 suites starting at around $700 per night and rising to $2,000 or so.
Next would be Nobu, with 181 suites, a drop in the Caesars Palace 4,000-room bucket, and rates as low as $229 mid-week, going to $1,599 weekends.
The Sky Suites at Aria comprise 310 suites; prices start at $900 for a one-bedroom penthouse with Strip view, $2,900 weekends.