This is Spinal Tap. The cast does the commentary in character, as if they’d just discovered many years later that Rob Reiner was making fun of them rather than it being the serious documentary they thought it was. They spend the whole movie talking shit about him and it’s hilarious.
Anvil: the Story of Anvil. Such a touching movie by itself, the commentary by the director, a childhood friend of the band, adds more depth and pathos to the story. My wife and I watched the movie and then rewatched it immediately with the commentary even though it was past midnight and we would normally be in bed. The story is just that gripping.
Second the mention of Tropic Thunder made in another comment. RDJ improvising the whole commentary in character is amazing and will increase your estimation of him as an actor, if that’s even possible.
Nirvana famously said they knew they had made it when Weird Al did a parody of Smells Like Teen Spirit.