Rayo's Number
The smallest number bigger than any finite number named by an expression in the language of first-order set theory with a googol symbols or less.
Now everyone knows that it is the largest number till date. But I've heard of some numbers that are even larger than Rayo's Number. The two of them are Fish number $7$ and BigFoot. I couldn't find their wikipedia so I can't attach their definitions.
Which number of three of them is the biggest $?$ Also, if someone can explain me the definition of Rayo's Number it would be greatly appreciated.