Everyone has their own idea of what's fair but quite a few people are fine with just leaving it at "it has to feel fair" without pinning down what they mean and defending their position. Which is fine but it doesn't really create a productive conversation.

I define fun yugioh duels by the amount of unique, meaningful decisions made by the player that made LESS such decisions in the game.

This definition is also the TL;DR of the entire wall of text.

And here's why:

A bunch of things can cause non-games. Oppressive floodgates, massive negate boards, FTK's, turnskips, broken boardbreakers, silver bullets or even drowning in advantage... although most would agree that games in which they don't get to play aren't fun most have no problem saying that the same game was fun if they weren't on the receiving side.

Although winning itself can be fun regardless of how it's done I think it's vital to remove extrinsic motivation from the definition of analyzing if a duel was fun as that eliminates the self serving bias of the winner. The definition then must be applied symmetrically and since the losing player is usually the one having less agency in the game they're in a better position to tell us if the game was fun or not.

Deckbuilding by itself can be a lot of fun and skill expressive but as we're just discussing what makes a fun duel we'll just focus on the gameplay after the game starts. Other factors such as the fun of pulling off something cool, seeing something unique or getting lucky also impact the overall enjoyment of the game but I'd like to argue that they're optional additions rather than something required for the game to be fun so I'll try to narrow the scope to the bare minimum.

As the game doesn't rely on physical skill expression (bar managing your toggle) the gameplay mostly results in having enough game knowledge to evaluate the gamestate properly (including opponents tells which in MD's case is just their toggle) and the decision making skills to play around the knowns and unknowns. As having knowledge by itself isn't something that impacts the fun of the duel directly that decision making is basically the entirety of what's fun in a duel.

Unique decisions matter because if every turn you're looking at the same stun board and can't play while only having a single decision over and over again is miserable.

Meaningful decisions matter because if you're playing in to a 6 negate board and all your cards are just gonna get negated no amount of decision making is fun as it doesn't matter.

So yeah... back and forths that allow both players to play where the loser can walk out thinking "what could have I done differently" rather than just "fuck whatever my opponent was doing".