I mean, if any is right:
The least you could have done is have won.
The least you could have done is won.
The least you could have done was won.
The least you could have done was win.
The least you could have done was had won.
The least you could have done was have won.
Also, could you teach me how to use this kind of structure? Just a simple explanation would suffice.
The only correct one is
The least you could have done was won.
could have is talking about the past and was is past tense, so it's the only correct one.
This one
The least you could have done was have won.
while maybe a correct sentence, just does not flow naturally. No need for the last have.
Just remember that was should be used when talking about the past. Is should be used when talking about the present or future. For example "The test is going to be hard on Monday." is talking about the future. And "The house is blue with a pink door." is present tense.
