Value wise, any one of the '64 halves that I turned up in my first box (beginner's luck!)
In terms of fun, I've had a lot of very fun finds in my nickels:
1994-P/1973-D
Magician's coin4 full rolls of a dumped collection, 1938-1959
1 full roll of mostly dateless
Buffalo nickels1891
V nickel1950-D (nothing like finding the key date after 3 years of searching)
Several nearly-uncirculated coins from the early 40's
When silver hit nearly $50/oz about a year and a half ago, I did the math and found that I had nearly $400 worth of silver that I had gotten at face value, but decided that I would rather hang on to it to pass down to my children and their children someday.
I never found any error coins, except for a couple greased quarters and a nickel.