I wasn't able to list every film he ever made on a poll limited to 25 options, so I've selected what I believe to be his 24 most known titles and left the last spot for Other choices. I based this list of the amount of total ratings for each film on RYM.

I still have many more I want to see, but so far my choice is between Rope, Rear Window, North by Northwest and Psycho. I'm leaning toward Rear Window for now. The other three I have seen are The 39 Steps, Vertigo and The Birds, all of which I would rank only a tier below the other four films. 

Incidentally, The 39 Steps is currently the oldest movie I have seen and it's single-handed converted me from ignoring older movies to actively wanting to see more of them. I had watched the western Stagecoach (1939) not that long ago, but found the film quality poorer. 

I think Hitchcock despite only seeing my first of his films last month has already become one of my favourite directors. 

There was a season of Hitchcock films on TV when I was a teenager. Psycho is still the one which sticks in my mind the best. Along with The Birds, I love how the initial story is completely hijacked by outside events. 

I must check out Hitchcock's version of The 39 Steps. I saw a TV movie version at high school, and have not long read the book. 

I'd also highly recommend Vertigo, Rear Window, The Man Who Knew Too Much, and North By North West. The others I either haven't seen, or I'm a bit hazy on.

His 39 Steps is very good. Dated obviously, but still very worthwhile viewing. I don't see any point in checking out any later versions of the story from 1959, 1978 or 2008 (I've heard this one is really bad). 

I see you haven't seen Frenzy yet Adam. That's deffintely worth a watch.

I currently have waiting to watch on DVD: Dial M For Murder, Shadow of a Doubt and The Man Who Knew Too Much (the second one). Plus a few others ordered or bidding on and a few lined up on Prime to watch. 

As in 'seen it but don't remember it' or 'don't remember seeing it at all'? Either way, it's well worth a pick up. I got a copy off of ebay (for free in the end, since the disc was badly scratched despite being sold as Very Good, even though it played fine the seller returned my money in full). Henry Fonda plays a man wrongly accused of theft and assault and must prove his innocence, while the ordeal takes a psychological toll on his wife's sanity. The scary thing about the movie is that the case is all true, which is what made it compelling viewing for me. 

Don't remember seeing it at all and I'm pretty sure I've seen most of his films at some point or other. They're not such regular fixtures on Tv as they once were.

I'm up to 17 of his movies seen now. I still have Marnie (1964) and Frenzy (1972) on DVD to watch plus some earlier ones on Prime, the one's I'm most interested in being The Lady Vanishes (1938) and Stage Fright (1950), but there's also Young and Innocent (1937) and Jamaica Inn (1939) on there. 

Just watched Frenzy - brilliant again! I hadn't realised Hitch made another British film so late in his career until it got going. 

He has a few duds in his filmography and I don't much care for silent films in general (though I hear is The Lodger: A Story of the London Fog is very good) but otherwise I echo the thought that any Hitchcock film is worth checking out. Especially from the 1940s onward with a few choice picks from the 1930s while Psycho should be a staple of any horror and thriller fan's movie collection. For me though, I hold a fondness for Strangers on a Train, which is one of my favourite old movies. Rope is excellent as well, though Rear Window is the best of the Jimmy Stewart collaborations for me. I never got the focus on Vertigo for that partnership to be honest. A good film but not a favourite of mine.

If you enjoy horror, then Psycho is a must.

If you prefer a crime or thriller movie, then try Rope, Strangers on a Train, Rear Window or Dial M for Murder. Actually any of his ones with James Stewart in are good entry points. 

His spy movies also have a few gems, such as North by Northwest. That's actually probably his most 'fun' movie to watch and a good entry point IMO. Notorious is another good earlier spy one, along with The 39 Steps. 

He actually has quite a few romance themed movies too, like Rebecca, To Catch a Thief and Marnie, but there's always something else going on in a Hitchcock romance, sometimes quite sinister. 

39 Steps is a pretty close second though, and then maybe Dial M for Murder and Torn Curtain.