Sooooo many to choose from. Usually you have to choose between the spoofs, the knockoffs, the serious spy dramas and all the foreign films.

At the top of the heap you have the Harry Palmer films which are three serious spy dramas that have completely different styles form the other. The Ipcress File is a masterpiece and the best serious spy film of all IMO.
The two Flint films are loads of fun and essentially a witty take on the 60’s spy craze.
The 1967 Casino Royale spoof is a madcap, wtf insane production that has an overloading embarrassment of riches and you can’t take all of it in in only one viewing.

We did a spy knockoff and spoof themed episode of the Laserdisc wolfpack podcast a little while ago: https://www.youtube.com/watch?v=V9XSc7KrBpE&list=PL1x8jwqGXR0j_x_6Ey1pxBF2sMT9T7eQw&index=23