What nation has best espionage-centered national ideas?

[Spude]

spy network construction, privateering etc. Popped to my mind and thought it'd be nice change to try that.

[Tulduil Iphukiir]

For pure Espionage probably
https://eu4.paradoxwikis.com/Samtskhe

Has +Rebel Support Efficiency, +Spy Network Construction and an additional Diplomat.

With Espionage, Offensive, Economic and Humanist you should get
+80% Spy Network COnstruction
+150% Rebel Support Efficiency

[cathartic]

Probably the Italian minors like Lucca and Ferrara. They get +10% spy network construction in their national ideas.

Another interesting option is The Palatinate, which gets +20% rebel support efficiency.