An object that is better than all of the other ones in it's category.
In Home Alone as Harry was driving along showing Marv the neighborhood he had checked out for houses to rob, they pulled out to the McCallister residence Harry: pointing to the McCallister house
That's the one, Marv, that's the silver tuna.
Marv: Oh, it's very gee.
Harry: Very gee, huh? It's loaded. It's got lot's of top-flight goods. Stereos, VCRs...
Harry: Probably looking at some very fine jewelry. Possible cash hoard. Odd marketable securities... Who knows. It's a gem. Hand me a crow bar. Crow bars up.