The Tu-95 and B-52 both had their first flights in 1952, both became operational in 56, and both are still operational today, with maybe plans to stay operational for ANOTHER 20 years.
So i'd guess it'd come down to a choice between those two, afterall they have stood the test of time.
I'd pick the Buff, the B-52.
Just think of that, a weapon system staying in use 80 years.
How long did some of the great fighting sailing ships of the 18th and 19th century stay in use?