gistfile1.py

>>> l
[(1, 1, 4), (3, 1, 11), (2, 1, 13), (6, 1, 17), (5, 1, 17), (4, 1, 18), (2, 2, 28), (1, 2, 29), (3, 2, 30), (4, 2, 31), (5, 2, 31), (6, 2, 33), (1, 3, 45), (3, 3, 51)]
>>> d = {}
>>> for x in l:
...     if x[0] in d:
...             if x[1] > d[x[0]][0]:
...                     d[x[0]] = x[1:]
...     else:
...             d[x[0]] = x[1:]
... 
>>> d.items()
[(1, (3, 45)), (2, (2, 28)), (3, (3, 51)), (4, (2, 31)), (5, (2, 31)), (6, (2, 33))]