Your personal best/favorite, basically. From what i can gather about the series, people are torn after 1, 2, 3 and 4.
As for me, well i'll reserve that judgement until i finish at least 1 - 4. My plan is to play the entire series(which is going to take a long time), and i started with Silent Hill 2 on my original Xbox, which i recently finished (excellent game, very memorable and one that i am going to replay wen i'm finished with a couple of more entries).
My 2nd game is the original Silent Hill on my PS1, in which i'm 30 minutes in. First impressions.. far harder to get into than Silent Hill 2 because of the tank controls and much more clunkier combat. I also feel that the game is harder than Silent Hill 2, the dogs and the flying enemies are a real pain in the ass. Got used to it though, and am definitely going to finish it before moving on.