The best way to experience them is to play them in sequence from the original PSX game, and then on to the PS2 games, but if you only want to play games with PS2 quality graphics then give Silent Hill 2 a try.
I personally still like the original Playstation game the most as it contained a great balance of combat, horror and puzzle solving. It was all quite logical and not easy to get lost of stuck. It also had some interesting alternative endings. I think one issue with the later games was that it was sometimes easy to find yourself at a point in the game not knowing what to do or which direction to travel and having to just experiment and try everything until you discovered something that moved the story on. This never happened (for me) in the original first game.
Trust us, you tried the worst of the series with the 4th game and it doesn't convey the quality of the others. I personally thought The Room was a good game, but it didn't stick to the same structure or ideas of the previous games.
One of the greatest features of the Silent Hill games is the shift of reality. You are never prepared or pre-warned that it is about to happen. One moment you could be walking down a perfectly normal deserted city street with a eerie sense that something is not right, feeling uncomfortable with your radio gently hissing in the background and strange sounds in the distance, then suddenly without warning the radio static would increase and the street would shift into the hell dimension where everything is the same but different, replaced with rusting structures and dismembered bodies hanging on metal fences, the sounds of things shifting around and crawling towards you, disfigured forms that once resembled people, hospital equipment discarded and covered in blood and other dried bodily fluids. Very gross! And the sound of industrial machines and boilers working away.
If you want to experience some of the best horror depicted in a game to date you definitely have to give SH2 a try, or even SH1 if you don't mind the Playstation one graphics.
Another aspect of the earlier games that were actually set in Silent Hill itself is the fog. This adds a great sense of apprehension because you cannot see far ahead of your current location when walking in the city. Not knowing what is just out of sight really added to the tension. And when you were injured and the controller was recreating your increased heart rate that also really added to the anxiety of the environments. It's also a requirement to play this game late at night with all of the lights out and a good surround sound system connected and turned up.
The music in this series is also really good.