Here's my opinion on the ones I've played:
SH1: Great for its time, and pretty much everything but the graphics hold up well. If you're going to play a bunch of them, do this one first. It's difficult to go back to after getting used to the improvements in the later ones.
SH2: Generally seems to be the favorite among fans - this was one of the earlier horror games I ever played (and one of the first games I played for my PS2) and it stuck with me as being absolutely brilliant years later. Great stuff.
SH3: I don't understand why there's negativity toward this one. It has some of my absolute favorite moments of the entire series (one involving a mirror is BRILLIANT). Definitely play 1 first since the plot ties directly to it and will resonate more with you if you know the characters.
SH4: Found this one to be an absolute chore to play through. Enemy designs took a dive, there's repeating areas where things are even less scary the second time, and the plot is kind of nonsense. Disappointing after the first three.
SH Origins: Played the PS2 port of this, which turned out rather nicely. I'm with Amazingu on this one (actually agree with everything in that post, generally) - this is the most underrated game in the franchise. The problem is that it doesn't really set out to do anything NEW. It's a solid SH game that's inoffensive and enjoyable, but does little to stand out to be better than 1, 2, or 3. Feels the closest to the original trilogy though.
SH Homecoming: Awful. There are game-breaking glitches that can force you to restart if you don't have a save to go back to and try to avoid them. It also degenerates into the torture/gore-porn style of horror that seems popular in movies lately. Way less psychological and way more "hey let's drill through somebody's face on screen - that's certainly scarier than the unknown!" Ugh. By far my least favorite of the ones I've played.
SH Shattered Memories: I LOVE this game and consider it one of the Wii's best. It really goes beyond what video games normally attempt to do story-wise, and it's a damn shame so few people played it. I've played through it a couple times with different people watching and watched others play it - it's a great audience game (with my friends most horror games are, though) and you see different things in every playthrough. All that said, though, it is a VERY different take on Silent Hill, and it's extraordinarily difficult to compare it to the rest of the series. It's best viewed as its own entity.