gistfile1.py

>>> def F(n):
...     x = 1
...     while True:
...         if n == 1: return x
...         elif n % 2 == 0: n /= 2
...         else: n = n * 3 + 1
...         x += 1
>>> input = 1429
>>> max(range(1, input+1), key=F)
1161