I'm not that guy, but I think it's easy to see some RPGs are masterpieces of TT RPG design qua design:
MASKS
Heart (more than Spire - despite me having run Spire so much more)
Lancer
Mork Borg (though I am not personally a fan, I am impressed with it)
Maybe Dread? Probably at least one FitD game (probably not Blades in the Dark itself, it's important but I don't think it was a masterpiece - I wouldn't put Apocalypse World on the list either, for similar reason). Shadowdark might be? I haven't played it - I have like, personal objections to some of what it's doing but what I cannot honestly say is that those stop it being a masterpiece - on the contrary they may contribute to it being one! Maybe Torchbearer? I haven't played it enough to say. Probably some other PtbA games (definitely not Dungeon World or City of Mists, though!). Maybe one of the Travellers?
There will be so many other I'm just not immediately thinking of. For me a masterpiece is a game where everything lines up, where there's a really coherent vision that produces something truly remarkable, and where it also actually plays really well at the table, and plays in a way that doesn't contradict that vision.
What a masterpiece is not, for me, is something that's important but not actually all that well-designed (Call of Cthulhu, for example, is important, but quite naive and unimaginative design-wise, and has elements that work directly against its thesis/concept). Nor is a masterpiece necessarily related to popularity. Popularity neither detracts from nor adds to whether something is a masterpiece in my view - you can see this in painting, for example - whilst a lot of the most popular and striking pieces in the world are masterpieces, there are a lot of masterpieces that relatively lesser-known, and it's always a delight when one comes across one for the first time!
5E is a jolly decent TT RPG, it's easily a 7/10 on the worst, meanest-spirited day and could be a lot higher. It's highly accessible, especially for something that is actually quite complex, rules-wise. But it has a fundamentally incoherent and self-contradictory vision, and a lot of smaller rules elements run directly against the over all sweep, and by trying to be all things to all men (or at least presenting as such), that inconsistency is amplified. That doesn't make it a bad game - for some groups it'll be the best game they play, and they'll love it forever. But it's not a masterpiece, because of incoherency, the fact that the 2014 edition was both rushed and designed as an "apology edition", rather than one with a coherent vision. Either 4E or 5E could have been a masterpiece if either had a stronger vision and say, another year in the oven.