I will use Busy beaver function (BB in short):

BB(BB(BB(BB(BB(BB(BB(BB(999))))))))

Edit:

Busy beaver function is the fastest growing function known till date.
As written in the reference given below, the number of sand like particles in the entire universe is around 10^63; BB(1000) itself is greater than 10^63 raised to 10^63, 10^63 times. Can you even imagine how big this number is?
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Reference:
Who Can Name the Bigger Number?