Is The Ipcress File the best spy film ever? Here is some convincing evidence (contains plot spoilers):

Palmer is the antidote to Bond

James Bond is a spy whose catchphrase is his own name - we know this is ridiculous, but we choose to ignore it. This is why it is refreshing that Palmer is more of a secret agent; less glamourous, but entirely more believable. We are therefore sucked into the undercover world where the danger is from exposure, not explosions. Courtney: "Do you always wear your glasses?"
Palmer: "Yes - except in bed." When Bond is captured he is strapped to a laser-table, Palmer just freezes in a cell. Bond villains are parodied for trying yet failing to kill 007 - "Begin the unnecessarily slow dipping mechanism!" Real evil organizations know the immense worth of double agents, framing, decoys, and, if pushed, no-frills assassination. There is an important similarity, though; James doesn't have the monopoly on the girls. After all, our hero is into books, music and cooking but "likes birds best".

The script is lovingly detailed

It is a strange turnaround when a novel has more action than the film, but it is justified in this case. The writers resisted the temptation of Bond-style globe trotting, opting for a tight London-based story. Palmer: "Er, is that my B107, sir?"
Ross: "As if you didn't know. And it makes awful reading, Palmer." The characters are also toned down, adding consistency. To pile on the paranoia, working-class Palmer is treated with condescension at all times; the book's seedy Bluejay is transformed into a creepy professor ("But you'll learn, my dear boy") and action-man Dalby is made an up-tight sub-Ross ("Close the door, Palmer").

Deighton novels are filled with accurate detail and it is pleasing that a fair amount survives the transfer to film (some more "sensitive" material didn't even make it to the novel). There are the now familiar hallmarks of spy films; microfilm, grainy briefing footage, businesses as fronts, Eastern Europe, underground carpark exchanges, bowler hats and umbrellas. But Ipcress goes one step further with glorious jargon such as:

We are left to crack this code, and so a little clarity is exchanged for a large dollop of authenticity. (Even the title includes an impenetrable made-up word). Remarkably, the science still holds up - the "important piece of scientific equipment" was working on nuclear fusion, which is still an active research area today.

Expert character acting

Caine's star-making performance is backed-up by marvelously convincing support players. Palmer: "If Radcliffe had been here, I'd have been a hero"
Dalby: "He wasn't. And you're not." Doleman's Ross has just the right amount of disinterest and manages to match Palmer in the dry sarcasm department. Green's Dalby quietly seethes under the parade-ground manner. The filmmakers almost give him a theme-tune with the bandstand and marching band backgrounds. Gatliff's Bluejay is deliciously self-satisfied with a slow, creepy delivery. And Lloyd's Courtney is certainly no vapid Bond-girl - she does her share of actual undercover operations.

The look is distinctive and original

It has to be said that director Sidney J. Furie had problems with the script, clashed with Saltzman, and was thrown out of the editing room. But these problems certainly didn't stop him infusing the movie with ultra-stylish camera angles and original visual ideas. Dalby: "I think they're playing rather well."
Palmer: "Tell me who wins." Immediately apparent are the shots looking through objects - a pair of glasses, a keyhole, a chair back. The shot through the prison door creates an impressive improvised split-screen. The shots through mid-height desk objects anticipate the now-familiar secret filming techniques, increasing tension. We are also treated to cool noir tilts and even a few fast zooms. POV is used twice for cars and the first-person-shooter finale. It goes without saying that this film must be viewed in widescreen; there are some scenes where the entire frame is used. Characters lurk in the background right at the edge of the screen (the CIA man in the library, Palmer in the warehouse).

The score is a masterpiece

John Barry need not fret about not actually writing the Bond theme as the Ipcress music has also become a classic. Barney: "And if you're not clean, I'm going to kill you."
It is still used today to instantly signify "Cold War Spying". Barry cites as inspiration the zither music from The Third Man, another iconic score. Unusual instruments, jazz, many separate themes, subtle, a masterclass in film scoring. Just listen to how the trumpet cue intensifies the dead-agent-in-flat scene.

Actual espionage is the main theme

Ipcress is packed with real spy stuff: surveillance, tailing, secret meeting places ("T108", a park bench), Dalby: "Incidentally, the Americans have put a tail on Palmer."
Ross: "How very tiresome of them." sniping, fronts, shady deals, moving in with a friend when you're in danger, CIA, War Office and Home Office, identity theft, code words, records with 'secret' written on them. The underground car-park exchange ritual debuted here, and was subsequently copied by many other films.

We are shown a brainwashing

Even the filmmakers now admit the method pictured is a little naive compared to the authentic methods portrayed in the book, but this key scene still does its job, and is tightly integrated into the plot. Palmer: "...I've read the file."
Grantby: "I know. That's why you're here."
The crossed girders are seen in the first warehouse, likewise the sound on the tape. The hand wound and the hypnosis trigger are set up, and the "softening-up" in the cell is a very real technique. This all needs a highly visual backdrop, which admittedly is a little psychedelic. But it was the 60's, and no-one was complaining about the end of 2001...

The whole scene plays like one of the more satisfying Mission Impossible TV shows merged with the intelligence of The Prisoner. Best of all, we get to hear Caine shout out the rousing line:

"My name is Harry Palmer!"