Marx Soul in Kirby Super Star (Ultra).
Because from what I recall, there was a cutscene before that, and a silence for quite a few seconds... then as you can hear, that music starts right away and you obviously jump. That's how that boss came as a surprise.
And that music is irregular, so it adds to the creepiness of the boss.
Plus, it's not necessarily made on purpose but maybe a consequence of the SNES capabilities, but the volume on that music is constant, kind of like the pressure this guy puts on you. He comes from below, both sides and above, you have to be focused at all times (well ok, my sister and I were young when we played that game so maybe we weren't the best players either...).
The Gogmazios in MH4U/G.
I love how the first theme is kind eerie and describes pretty well how a hunter must feel when you see that skycraper monster in front of you after being told "finish what the dragonator couldn't" lol. You're like "ok, this is my last quest, either because there is nothing after that or because I'll die"

Then comes phase 2, with a music I don't really like... except for the fact that you can interrupt it with what I'd call the official Monster Hunter theme if you manage to use the Demolisher properly. Yeah, that's right, 3 musics for one monster only

And that last one has the opposite effect compared to the first one. You're now convinced you can do it.
The Chaos Gore Magala in MH4U/G.
Same as the Marx Soul, the music isn't the most regular one on earth and it also provides some of these feelings we find in the Gogmazios theme, like "am I really trying to take this down?", even though here it sounds more impressive than impossible.
But what I like most is how this guy is some sort of failed evolution of a Gore into a Shagaru, and how its theme is an eerie mix of the Gore theme and the Shagaru theme