This is an interesting card. My first thought was that it was potentially too easy to kill when you just throw it onto the battlefield. However, the upside here is extremely high. If your opponent is unable to remove Xanathar right away, we have the potential to be able to lock our opponent out of the game.
You presumably are playing a mana-heavy deck to have Xanathar in it, so you can try to sort of leave lands on top of your opponents deck every turn when you pass turn, or even just leave low impact cards on top. Various cards will work against various decks. Again, the biggest risk is that your opponent just removes Xanathar right when you play it, which is a big risk, especially because you’re playing a six-mana card that doesn’t have any immediate effect.
However, if you ever untap once, that likely means that your opponent doesn’t have any immediate way to deal with Xanathar, and you can likely engineer the remainder of the game in a way where they’ll continue to not have an answer. One thing that could cause an issue here is that if there are no other creatures on the battlefield, you can’t actually spend a removal spell from the top of your opponent’s library. This might not be the most common situation though, particularly against creature-heavy decks.
Against Adventure decks that might have something like Brazen Borrower or Giant Killer on top of their library, you can just cast the creature half of the card. You’re also drawing cards off of your own deck every turn, so you can freely allow your opponents to draw cards that you have an answer to.
Against decks with a ton of cheap cards, like Mono-Red, you should be able to just completely control the game. For a deck like Sultai, it could be a bit more difficult. When the top card is something like Binding the Old Gods, and the next card is Emergent Ultimatum, it may be challenging to get through multiple cards in a turn. Of course, there are other cards you can use to manipulate the top of your opponent’s library, most likely mill cards, but I’m not sure any of those cards will be worth it.
Overall, the upside of Xanathar is extremely high, almost game-winningly high. It’ll be interesting to see if a six-mana 5/6, even with massive one-turn-later upside, will be playable in a time where most cards over three or four mana typically need a pretty big and immediate impact to be worth putting in a deck.