Warning: looooooooooooooong post ahead.
There are 18 cards I buy over 85 percent of the time, twelve of which are over 90. That's a lot of cards, which indicates that perhaps I am more predictable than I ought to be. While there are no cards which are true "must-buys", a couple come close- and no I don't think Chapel comes closest.
#1 is Fishing Village, which I buy 96.9 percent of the time. Considering it's the #1 bought card on Isotropic overall, that's not actually surprising. By itself, it's not a must-buy, but there are vanishingly few situations where it isn't strictly better than Silver (since it gives that much money anyway in addition to the extra actions). It's pretty close to being a must-buy.
#2 through #5 are all Cornucopia cards! Hunting Party at 95.5, Menagerie at 95, Tournament at 94.3, and Remake at 93.8. There is definitely an "ooh shiny! I want the new cards!" effect going on, but it's actually a really small effect. HP is so much better than Lab it's not funny- a stack of Hunting Parties doesn't just get you a huge hand each turn, it is especially good at enabling tricky combos or working with untrimmed decks. There was a game a couple days ago where I skipped HP because I had a double-Tactician setup with trashing and didn't need any other card draw, but HP is just so good in such a wide range of situations. I might even rank it above Minion, Torturer, and GS in a combined Top $5 Cards list. Menagerie does no harm even in decks it's ill-suited for, and does so much good in favorable situations. I probably overbuy it a little, but not much. Prizes are powerful enough that I'd only consider skipping Tournament in games which are obviously suited to Goons or Gardens, and Remake is another card I probably overbuy but it's a good enough trasher that I'm willing to take it instead of Chapel half of the time.
#6 is Ambassador at 93.6 and honestly I'm not sure why it's that low. If you're not going Gardens, or it's a Possession game with other strong trashing options, you need to go Ambassador and win that war, otherwise you're likely screwed. I consider Ambassador to be the closest thing Dominion has to a true must-buy, and would absolutely rank it above Chapel as "best card in the game". BTW I utterly hate Ambassador games.
#7 is Festival at 92.3, which seems way too high until you realize it's way too high for Isotropic at large, too- not surprising considering it's really just an overpriced Fishing Village with +Buy. (Given how good FV is, this makes it not actually overpriced.) I have a huge soft spot for Festival and do very, very well with it around, but wouldn't call it a must-buy.
Five other cards break the 90% barrier for me: Goons and Hamlet at 91.5, Nobles at 91.3, Chapel at 90.8 and Mountebank at 90.6. Here we're finally getting to the sorts of cards which most people would call "must-buy". Goons is pretty close- even if you can't build a Goons engine it's probably a good idea to make it your terminal; Mountebank is the baddest attack out there; Chapel is Chapel. Hamlet is an ubercheap "do-no-harm" cantrip with surprisingly strong powers to enable chaining, and Nobles is a card I definitely overbuy but still think is important to grab your share of in most games.
The rest of the over-85 crowd include King's Court at 88.8, Apprentice at 87.3, Mining Village at 86.8, Minion and Upgrade at 85.5, and Caravan at 85.4. Of these cards, King's Court is the closest to a must-buy- unless you're playing a super-Cursy or Gardens game where nobody ever reaches $7, you're going to need some of these. It's the most powerful card in the game, and its ability is arguably "unsafe at any price". There was a period of time where I overbought Minion, but it's usually a must-buy except in certain well-defined circumstances. Apprentice is rocket fuel for your deck, it's almost always worth buying. Mining Village and Caravan do no harm and usually do a lot of good, and Upgrade is a card I massively overvalued for awhile. Well, actually I'm not sure Caravan does a lot of good: I do horribly when I buy Caravan and it has by far my worst "Effect With" at -3.47.
…
There are some surprising omissions! No Familiar, no Witch, no Province. To be fair, Familiar and Province are right behind. Familiar is kinda like Chapel: if it's out you need to get a card that does what it does, but you don't necessarily need that card itself. If the other Potion pickings are slim, I'll pick a different curse-giver. Witch I should buy more, but again it's not my favorite curse-giver: I'll grab a Mountebank first almost every time. Given my play-style, I kinda expected Tactician and Peddler to be higher as well. If you just looked at the past couple months Masquerade would also be up there, but I ignored it way too often when I was starting out here, so it has a hill to climb. Not sure why Grand Market is so low either.
…
Alternatively, instead of looking at our Buy%, why don't we look at our "Win Rate Without" and pick the cards with the worst rate- the cards that, if we don't buy, we lose. On this list, my champion is the humble Hamlet, which if I skip it leaves me with a terribad 0.71 Win Rate. Hamlet! Whoda thunk! Second is Fishing Village at 0.78, which lends credence to the idea that these cheap cards aren't just "do-no-harm", that you in fact can't afford to ignore them and still win. Third, at 0.89, is Trusty Steed, but prizes don't really count here. Next is the Best Attack, Mountebank, at 0.92. Fifth is Lighthouse at 0.95. Yeah, Lighthouse. Which is surprising until you realize it's a Silver's worth of buying power for $2, and has the strongest Reaction effect in the game. Perhaps it is really a must-buy when attacks are around, as opposed to Moat and Secret Chamber which are terrible cards that should usually be avoided even when attacks are around.
There are three other cards where I only break even when I skip them. Two of them, Chapel and Ambassador, should be no surprise. The third is Woodcutter. Why I do so poorly when I skip Woodcutter is an utter mystery, probably even more so than why I suck at Caravans.
All in all, the top 10 cards I consider closest to being must-buys, in rough order:
Ambassador
King's Court
Mountebank
Fishing Village
Goons
Familiar
Hunting Party
Tournament
Chapel- yes, all the way down here at number 9.
Hamlet
The next cards looking in would be Witch, Masquerade, and, I dunno, Grand Market?