Is there an idiom to express "You couldn't get anything better"?

Solution 1:

"Suit yourself" is a nice quip to indicate acceptance of the other speaker's preference. Depending on your tone, this short phrase might also be a polite indication of disappointment.

If your aim is to emphasize your disappointment, I agree with Rathony as to saying "You don't know what you're missing."

Finally, if you want to be a bit rude in turn, you might just say "Your loss."

Solution 2:

You could consider using You don't know what you are missing.

It means you will regret if you don't accept my offer or proposal. It could be rephrased to:

You don't have any idea how great my offer would be and you will regret if you miss this opportunity.

Solution 3:

Take it or leave it, that's my final offer.

would frequently be used in this situation.

"Junior doctors' leaders have rejected a "final take-it-or-leave-it" offer made by the government to settle the bitter contract dispute in England."

Solution 4:

In England we would simply reply, beggars can't be choosers.

said when you ​recognize that you must ​accept an ​offer or a ​situation because it is the only one ​available to you:

I would have ​preferred a ​house of my own ​rather than ​sharing, but ​beggars can't be choosers, I ​suppose.

_{Beggars can't be choosers. Retrieved February 26 2016 from Cambridge Dictionaries Online.}