to take the biscuit/cake

[Theseus]

To carry off the honours. Sometimes used to express incredulity; for example:-
-- "That's three goals he's missed in one game. If that don't take the cake...".

The related phrase 'take the biscuit' means virtually the same, although is more often associated with surprise at a particular outcome than with victory in some enterprise.
In UK slang you say that something or someone (really) takes the biscuit when they have done something that you find extremely annoying or surprising:-
--And you say she's opening your letters now? Oh, that really takes the biscuit!:curse:

 

[Hellegennes]

To express the meaning of being outstandingly good, "είναι και ο πρώτος/είναι κορυφή". For the negative meaning, you could say "πάει για βραβείο" or "είναι απ' τα άγραφα". For example:

I've seen my share of bad films, but this one takes the cake
Έχω δει κι έχω δει κακές ταινίες, αλλά αυτή πάει για βραβείο

I've heard ridiculous accounts, but this story takes the cake
Έχω ακούσει πολλές γελοίες ιστορίες, αλλά αυτό είναι απ' τ' άγραφα


As for the meaning of "to take the biscuit", could be:
Το παράκανε
Το παράχεσε (more blunt)
Το ποτήρι ξεχείλισε
Είναι άνω ποταμών
Δεν πάμε καλά

 

[azimuthios]

For take the biscuit and exaggeration there is more vulgar phrase: Το γάμησε και ψόφησε.