My favourite card right now might be Sphinx Ambassador.
To sum it up, it's the greatest mind game in Commander I can think of.
Before we go further, I'd like you to think of a Commander deck of yours. Do you know what the best creature in it is? Can you name the top three? Top five? In my playgroup, everybody runs multiple decks, so our memory of what's in them is less than photographic.
Now let's talk about the logical rabbit hole this creates: If I smacked you with this Sphinx and picked out a card, what would you say? It would make sense to not let me have the "best" creature, like an Elesh Norn, so you'd pick that, right? But knowing that, I would pick your second best creature. But you also know that so you should pick that one, right? But then you know that I know that you know I would do that so you're back to your first creature. Unless you only have two or three creatures in your deck, I don't see how you get out of this one. Not to mention all of this is dependent on whether or not we agree on what your "best" creature is! Ooh, I feel so smart playing this card!
"Never go in against a Sphinx Ambassador when death is on the line! Inconceivable!"
It can be a real blast to add some mind games into a match of multiplayer commander. It'll certainly lead to some interesting discussions after the game is over, which is part of the fun of our format!
Here are a handful of other cards that can lead to some really interesting mental struggles! Hopefully your opponents have it worse!
If everyone agrees to bid zero before the Goblin Game begins, we could all share the penalty with no risk of that extra hit from losing what we bid. That's what you suggest to the table before this resolves so that you can bid one and everyone else loses half their life! But I'm sure your opponents are smarter than that so one of them will bid two, which means one should bid three, and here we are in a dastardly logical loop again! You, the caster, are on the hook just as much as everyone else, but you get the pride in knowing you started it!
This one happens in real time, so you get to watch each player struggle with how much they're willing to bid for four cards. Standard payment is two life per card drawn, so eight life seems on par, right? Well, one more seems fair to bid. And maybe one more after that. Okay, so, maybe three life per card seems fair. Well, at this point what's one more life! Here we are again in the loop! I say try and cast this as early as possible so that each player can dig their own grave as deep as they want! It's way less fun when nobody can bid because you're the only player with a significant life total!
This one is less group fun and more picking on one player. Probably either the player with the most permanents or the player with the least life. Both will lead to unique groans and a likely enemy. Respect to the opponent who keeps all their permanents and goes down to a small handful of life points and tries to take you down with them. The only honourable way to go.
This one hurts my brain and I don't even know where to start. I love it and run it, but I also hate it. I hope it hurts others' brains as much as mine.
This one takes the longest to resolve for sure, and it's worth noting that in the oracle text, it doesn't affect tokens. Other than that, the game will look very different after resolution! The interesting choice here is, do you go for lands first to be able to keep playing, or grab the bombs? If one player is going for all the bomby creatures, we all have to keep up the arms race, right? Unless someone is scooping up lands. Does that mean they're holding a wrath effect? Now we all have to grab lands to not get blown out! Again, so interesting! So many difficult choices! The one thing I know for sure about Thieves' Auction? I'm taking Sphinx Ambassador as my first pick.
Do you have a favourite mind game card? Let's do that thing Mr. Burns does with his hands while we chat about it in the comments!
