Today I'd say
His Girl Friday
The Jungle Book (Disney)
North By Northwest
Butch Cassidy and the Sundance Kid
In my opinion each of these movies is as perfect as a movie can be in its respective genre, so I think they make a nice, short list to show my taste (and just for the record: I consider Butch & Sundance not as best western movie, but as best buddy movie ever made, if that counts as a genre).
My list would change depending on my mood and memory if you would ask next month or next year. I love at least 100 movies out of the several thousands I've seen so much that narrowing it down to my "real" top 5 would take forever and probably drive me mad, e.g.
"hm. His Girl Friday is a near perfect comedy and among my most favourite ones in that genre but do I really like it more than Philadelphia Story or Bringing Up Baby or Some Like It or Hot or The Front Page or A Night at the Opera or Duck Soup or Ninotchka or Annie Hall or Love and Death or The Great Dictator or The Flying Deuces or Silver Streak or Young Frankenstein or L’aile ou la cuisse or Those Magnificent Men in their Flying Machines or Groundhog Day or..."
...you see what I mean.