"What spy RPG have you enjoyed most? Give details."
Honestly, I've not yet been in a position to play a spy RPG as of yet. However, I've run some non-spy genre games that had aspects of espionage. The most memorable was a run in the Serenity RPG's Cortex system where the heroes had to infiltrate a private security compound and steal safe codes for a bumblebee-class freighter that had crash-landed on an outlying planet.
There was a good bit of sneaking around at night, climbing walls, breaking into buildings through air conditioning chutes, and disabling security devices, with light puzzle solving based on both character skill rolls and player ingenuity.
Lastly, though, was the daring escape with a stolen security skimmer that burned the faces off the night watchmen.
I really enjoy the ability to switch the play style of RPGs on the fly and find out how your players plan on running with things, though having a system dedicated to a particular play style is also a valid option. That said, I'm really interested in trying out Shadowrun 5th, and seeing what all the fuss is about.