What is the most commonly missed Heart Piece in the game people forget the most?
For me it was definitely these 3, despite the fact that I've beaten OOT like 40-50 times between the N64, the Gamecube ports, the Virtual Console, and the 3DS remake:
The heart piece in the random grotto in Hyrule field to the left of the castle entrance by a random tree. You really have no reason to travel over there at any point of the game, so I would always miss that one. And on the N64 you needed the rumble pak to tell you there was a grotto there, on the 3DS it's much easier since the stone of agony always chimes. It's just totally random.
The Heart Piece on the bottom of Zora's Fountain as adult Link. You see there are two heart pieces here, and everyone sees the one on the ice glaciers you jump to (to the right of the Ice Cavern entrance). But there's ANOTHER heart piece on the very bottom of the lake where Jabu Jabu used to be, and you literally have no reason to ever go down there otherwise with the Iron Boots. So I always miss it.
Playing all the songs for the two Skull Kids on the right can sometimes be overlooked, since you sometimes don't remember to do it.
Honorable mention is racing Dampe a second time for a Heart Piece in his grave, most people just race him once the first time and forget to race him a second time in the rematch.