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The most useful parts really depend on your style of building, especially if you are into microscale vs say minifig scale.
1x1 Round Tile
1x2 Masonry/Profile Bricks
1x1 ‘Headlight’ Brick really doesn't belong. It's not as handy as the flush mounted version.
There are more useful parts out that. 1X1 tiles, brick separators, 16X16 plates, different tiles, etc
And 16x16 plates. I reckon I use those in just about every build I do.
If you micro-build for example, the headlight brick will be useful as windows.
Typically, when people say "most useful", they really mean "most versatile". While some of the commenters here take "most useful" to mean "used most". So we get such funny suggestions as the 16×16 plate. (Come on, people, headlight bricks and cheese slopes can be, and are, used for literally everything from a microscale animal to the Tower Bridge; the 16×16 plate cannot be used for either.)
Likewise, nobody even defined what constitutes "a use" in the first place, so we get such funny suggestions as the brick separator.
Rebrickable actually has nice cold stats. That's yet another (perfectly valid) definition of "useful".
I do have to say, though, that "annoying to put on" is completely orthogonal to any definition of "useful" I can think of. Pants are annoying to put on, too. (I will also add that I have never once seen a cracked cheese slope, and don't find them annoying in the least, even though I have huge hands in point of fact.)
And then of course some people say "bricks" to mean "parts", while others take "bricks" to mean "bricks", while others still mean "only system bricks but not Technic ones", yadda yadda.
The way I see it, the brick separator is not even a part, the 16×16 plate has exactly one use, and the 1×1 round plate with open stud is simply way too new to say anything about it with confidence.
But again, that's just me. Until we clearly state what our criteria are, or at the very least give that list a less ambiguous title, all discussion is sort of pointless.
In the video for the Tower Bridge, Jamie Berard stated that his favorite parts were the headlight brick, the jumper plate, and the cheese slope. These would be my picks as well, but that's beside the point. The point is he called them a) "favorite", b) his favorite, and c) stopped at three. He is a wise man.
"Most" should serve as an invitation to come up with very few select things. "Ten most" is only asking for trouble.