Depends on the producer. Most of the plantings are Furmint. Second most, but a really distant second, would be Hárslevelű and an even more distant third would be Muscat, usually what they call Yellow Muscat or Sárga Muskotály, aka Muscat Lunel. That is one of the varieties of Muscat, which is more properly considered a family of grapes rather than a single variety. It’s not the Muscat found in the south of France.
There are a couple of other, really minor grapes, like Oremus, which is actually getting some further interest these days, but Furmint it far and away the main grape. Actually, that too is somewhat controversial. The communists planted a variety that has big berries, as they were interested in quantity. The more traditional variety has berries of different sizes, somewhat like the Wente Chardonnay clone, a characteristic UC Davis calls “hen and chicks”.
Anyhow, because so much communist era stuff was planted, some of it is now on old vines and considered decent, whereas some newer producers are finding and/or planting the older type.
In any event, the real answer is “it depends”. You’re not required to use the same grape. Probably most common would be either completely Furmint or whatever the blend is in the field. But there are many variants, I’ve had some that are 100% Hars, and 100% Muscat, although the latter is quite rare. And some will use the dessicated berries from one variety and a base wine from another, and as mentioned, depending on what they want to achieve, the wine can be in various stages of fermentation.
The question is whether you can really distinguish on the palate and that’s the million dollar question. I don’t think it’s all that easy, partly because the wine is so complex. Even Muscat, which is so noticable on its own, gets muted by the botrytis. If it’s used, it’s generally a small part of the blend anyway, but as you know, even 5% Muscat can produce a big effect on the nose.
On its own, Furmint produces a very dry, almost austere wine, very similar to a Muscadet from the Loire. While once discouraged, today some producers are putting out dry versions, as the market for sweet wines is so small, so you can actually find a few in the states if you’re interested. Most common, and probably the weakest of the lot, would be the wine put out by Oremus - Mandolas. I don’t think much of it, but it will give you some idea. Much better was the one put out by Kiralyudvar, but that’s seriously flawed in some cases so beware. Best value and a good wine on top of it is probably by Dereszla and in fact, their sweet wine is in the same category. For me, their lineup is actually much better than the RT, which is beloved mostly by British critics.