Clickbait subtitle: Number 6 will make your jaw drop!
While obviously Paul Darrow would have made the best Number 2 evar, let's be sensible cats and confine ourselves to actors that could have plausibly been about in the era that The Prisoner was being made, and well known enough that they could have realistically been cast as the series' second main character for an episode or two.
Patrick Troughton could easily have made the list but for one thing, which is that he spent the whole of the period in question being in Doctor Who as a full time job. He just misses out on my top 10 for that reason.
#10. Kevin Stoney
Kevin Stoney is a good enough actor that he could be placed higher, but he is let down by his performance from that time he was actually in The Prisoner, as Colonel Jolly-Good-Show-What-What in Chimes of Big Ben - a character so broad in his English mannerisms that he could easily be best friends with Biggles or Bertie Wooster.
But imagine if Kevin Stoney had played his Number 2 more like his Tobias Vaughn in The Invasion (1968), then we'd be purring.
#9. Anthony Hopkins
Making this list mainly on the strength of his work in the Department S episode A Small War of Nerves (1970), plus of course all the hindsight of his subsequent career, Anthony Hopkins only places so low because of the feeling he'd be wasted as Number 2.
In the Prisoner episode Do Not Forsake Me Oh My Darling, Nigel Stock has a good go at playing Number 6 trapped in another manny's body, but despite a couple of strong scenes he can't rescue the rubbish production from itself. Now imagine Anthony Hopkins filling those shoes instead, and I think he may have had more chance than anybody else (except for a time-travelling 1980s Paul Darrow) of lifting that story up.
#8. Dennis Alaba Peters
Staying with Department S for inspiration, Dennis Alaba Peters played the regular authority figure Sir Curtis Seretse, and I think this could have been a good template for a take on Number 2.
He just needed to stay away from dodgy back projection when meeting with
#7. Herbert Lom
Herbert Lom was mainly a film actor, but did appear in a few TV shows, such as playing the main character in The Human Jungle (1963-64) and a guest star in The Man From UNCLE in 1967.
Despite looking a little bit like Kenneth Griffith (Number 2 in The Girl Who Was Death and, arguably, Fall Out), I don't doubt that you would have gotten a very different performance out of this veteran character actor, a few years before he got a bit typecast from playing Chief Inspector Dreyfus in the Pink Panther films.
#6. Philip Madoc
Going by his performance as the War Lord in The War Games (1969), Philip Madoc could have been a compelling, sinister, threatening Number 2, a worthy antagonist for Number 6 to face.
Contractually obligated joke:
"Why did you resign?"
"Don't tell him Pike!"
#5. BRIAN BLESSED
BRIAN BLESSED's versatility and range as an actor have been somewhat obscured by his post-Flash Gordon persona, but if you look at his work in the 1960s and '70s, such as guest appearances in The Avengers or his Augustus in I Claudius, you see what he would have been capable of as a Number 2.
Maybe his beard is Number 1?
#4. Honor Blackman
After leaving The Avengers to star in Goldfinger (1964), Honor Blackman could have returned to telefantasy television as a Number 2 who was the dark side of Cathy Gale or Pussy Galore.
Naturally as a cat I am bound to say I'd prefer the latter option.
#3. David McCallum
The Man From UNCLE series ended in 1968, so it is just possible that David McCallum could have come straight from there to be in The Prisoner, a few years before playing a prisoner himself in Colditz.
David McCallum is a truly great actor, so his Number 2 would have been bound to have been memorable in the first place, but there would also be the added question if, just as some speculated that Number 6 was really John Drake from Danger Man, could his Number 2 be Illya Kuryakin?
#2 Christopher Lee
In between making Dracula sequels for Hammer* Christopher Lee guest-starred in The Avengers episodes Never, Never Say Die (1967) and The Interrogators (1969), so it is not inconceivable that he could have been in The Prisoner around that same time. I think it's pretty uncontroversial to say he would have been a magnificent Number 2.
* This isn't even a joke: Dracula Prince of Darkness (1966); Dracula Has Risen From the Grave (1968); Taste the Blood of Dracula (1970).
So who is #1?
You are Number 6.
Only joking, it's:
#1. Roger Delgado
Roger Delgado's list of acting credits on IMDB for the 1960s show him as being stuck playing shifty foreigners called Von Golling (Danger Man, 1961), Rodriguez (The Saint, 1966) or Kreer (The Avengers, 1969), and many more examples both on film and on television. Inevitably they were minor villains, henchmen or, at best, guest villains of the week.
Aside from a recurring role as the Spanish Ambassador Mendoza in Sir Francis Drake in the early '60s, it took until his (perfect) casting as the Master in Doctor Who for him to find a regular part that showed off his acting skills to the best of his considerable abilities - although the Master was an alien, Delgado did not have to put on a dodgy accent or even more dodgy facepaint in order to play him.
I think it would have been great if we had seen a Number 2 played in the same vein, if only the producers of The Prisoner could have looked past the typecasting to the undoubted talent underneath. With all the qualities that made Roger Delgado's Master such a brilliant foil for Jon Pertwee's Doctor, Number 6 might have met his match at last.
It would have been even better if they had allowed him to record his own version of the opening dialogue (rather than lazily use the default sequence, as they tended to do towards the end of the series). That way we could have had:
"I am the new Number 2... and you will obey me."