When I was little, we watched "Alfred Hitchcock Presents" religiously on TV, so I was well aware of Mr. Hitchcock and his reputation. "Psycho" and "The Birds" were getting a lot of press at the time, but I did not see them then. I probably saw a rerun of "39 Steps" or "Lifeboat" first. One of the first movies I really paid attention to of his was "Rear Window" which still holds up remarkably well. The first one of his I really waited to see was "Vertigo" and was worth the wait. "Man Who Knew Too Much" was probably one of the earlier ones I saw and still classic Hitchcock. I prefer this period of Hitchcock to "Rope" and "Strangers on a Train." Hitchcock was a master showman and really knew how to get publicity-- so even the later things (like "Psycho" and "The Birds" which, to me, are weaker movies) have so much hype about them even before you've even seen them.
"North by Northwest" is another of his best-- smooth, suave, cool and terrifying. Although I like "To Catch a Thief" for all its style and brilliance, I actually think "North by Northwest "is a better movie, and Cary Grant and Eva Marie Saint play so well off of each other.