I played against the Double Muzio twice and won both games.
Most dangerous gambit

Here's the more recent game. I sacrificed my queen to fend off the attack (honestly I don't think there's much of an attack at all in this position) and achieve an equal game, and I then won because my opponent blundered.

[From my point of view and in short]
a : The King's Gambit : no because 1) e4 e5 2) f4 Bc5
b : The Queen's Gambit : the openning is very strong but the story about dangerousness of the gambit with the annoyance of having lost to a weaker player because of a dangerous opening "that could scam", no and personally with White I am much more afraid of a semi-slave or a ragozin than Queen's Gambit accepted
c : The Blackmar-Diemer Gambit : "everyone says" it is strong but even if potentially dangerous I would say no not so much
d : The Evan's Gambit : yes it's not a joke, a real danger
e : The Latvian Gambit : the openning is weak even probably in the refuted category but otherwise yes because we don't know anything about it, our opponent is prepared and it is dangerous.
f : The Danish Gambit : not really in a way because simply 1) e4 e5 2) d4 exd4 3) c3 d3 but otherwise in a way like this gambit can be unbearable with danger yes in a way.
g : The Morra-Gambit : idem we can play simply 1) e4 c5 2) d4 cxd4 3) c3 d3 but after 3)... dxc3 once White has assimilated some tactical tricks based on Rook sacrifice etc. yes dangeurous and unbearable

I'll second the Latvian. It is NOT a reversed King's Gambit and I don't know how it's even CLASSIFIED as a gambit when the main line is just white wrecking your casting rights then chasing your queen with every piece he has while you develop NOTHING!
I scored some quick wins with it for a minute years ago, but soon, it was white stomping me more often than not. If that isn't a gambit that's dangerous for the gambiteer, I don't know what is, but there's gganother FUGLY gambit for black that 5 points down in a wrecked castle where black actually wins more than loses in a chess academy video
It's in his "5 surprising openings" video I can't find the URL for to copy with my TOY computer android tablet... the DURAS GAMBIT!!!!

I only am familiar with 1.e4 gambits, I don' know any Gambits where black cannot decline the gambit. I play the Danish Gambit as white , and 1.e4 e5 as black ,and accept all gambits offered.I would rephrase this question as " which gambit opening, in which black fully accepts the gambit and doesn't attempt to return material, is most difficult for black to play against?". Gambit lines aren't anymore difficult ,for black, than non-gambit lines if black chooses to decline them; same with gambit lines where black, at their initiative, returns some or all material taken to even the game. As black, I have played against all 1.e4 e5 gambits mentioned in this thread,I have found the Danish to be the most difficult to play against when accepted and black chooses to not play the Schlector( where endgame skill is a premium).I suspect that a significant number of players who play half-open defenses do so to avoid 1.e4 e5 gambits ( and the Ruy).

If we are talking about win rate:
Good for white:Latvian gambit, Queen‘s gambit, Petrosian gambit, Cordel gambit, Benko gambit, Mittenberger gambit,…
Good for black: King‘s gambit, Greko gambit, Smith-Morra gambit, Milner-Barry gambit, Belgrade gambit, Danish gambit, Blacmar-Diemer gambit,…
If we are talking about win rate:
Good for white:Latvian gambit, Queen‘s gambit, Petrosian gambit, Cordel gambit, Benko gambit, Mittenberger gambit,…
Good for black: King‘s gambit, Greko gambit, Smith-Morra gambit, Milner-Barry gambit, Belgrade gambit, Danish gambit, Blacmar-Diemer gambit,…
erm...are your colors mixed, or are you trying to prove gambits are bad? (some are, some aren't)

erm...are your colors mixed, or are you trying to prove gambits are bad? (some are, some aren't)
Colors are not mixed - that is the stats from masters games See for example 43% win for black vs 25% win for white - so it is certainly good for black

1. e4 c5 2. Nf3 nc6 3. d4 cxd4 4. Nxd4 e5 5. Nb5 d6 6. Be3 a6 7. N1c3! Axb5 8. Nd5! And it’s very dangerous for black
erm...are your colors mixed, or are you trying to prove gambits are bad? (some are, some aren't)
Colors are not mixed - that is the stats from masters games See for example 43% win for black vs 25% win for white - so it is certainly good for black
So you're trying to prove gambits are bad? because for the ones where white wins, you listed gambits black plays, and vice versa
heres a good muzio gambit I played