[btilly]

In a somewhat restricted version of this contest some time ago the best entry was my:

B(b, c, 0) = 0

i < b => B(b, c, i + j * b) = i + B(b, c, j) * c

T(b, 0) = b * b

T(b, n+1) = T(T(b, n), B(b, T(b, n), n))

Z = T(2, T(2, T(2, 9)))

It is much, much larger than the number you just threw out. See http://bentilly.blogspot.com/2010/03/large-numbers.html for an explanation.