Depends if you mean films of all types and eras or just modern films.
There are loads of 100 best movies if you search the web.
Here is one:
http://www.channel4.com/film/newsfeatures/micr osites/G/greatest/index.jsp
Can I suggest Citizen Kane which is considered by many critics to be the "best" film ever made.
Read up about it before you watch so you find out what is so special about it.
It was Orson Welles first film and he helped write it, directed it, and also starred in it. And he was only 25 years old.
http://www.imdb.com/title/tt0033467/
Even if you do not like it (and you may watch many you dont like) you still have to watch it to call yourself a film buff.