Hitman: Blood Money, Splinter Cell: Double Agent (xbox version) and Chaos Theory are probably some of the best stealth games around. In Blood Money, stealth is really more of a suggestion than a required path. In both Splinter Cells, it's always really obvious how visible your are, where enemies are at and when you're making noise. Not only that, you have a bunch of visual options that let you see any tricks or traps that might be ahead. Enemy AI is excellent. Sure, they have a patrol path or general area they stay in, but the guards have multiple alert level reactions and don't blindly walk into the dark if they can do something else.
Also, unless you grab a guy from the rear, all your ways to take out enemies normally involve making at least some noise. Another good thing about all three games is that getting spotted doesn't end the game, it just makes things harder.
Oh, and Chaos Theory had multiple paths through the levels. As did Blood Money. Blood Money even had some semi-sandbox levels. The Mardi Gras level is one of my favorite levels in all my years playing games.
Riddick isn't a bad game, but I don't know about calling a game where you're given a minigun and HAVE to use it in places a "stealth" game. It's more akin to something like Beyond Good and Evil, in that it melds a bunch of genres together. It's not really right in my opinion to take a "jack of all trades" game like Riddick and hold it up as an example/the pinnacle of any particular genre.
To big points I feel are essential to stealth games are graphics and control. Also sound, but graphics and control are the big two. See, stealth games tend to be kinda slow and tense compared to action genres. There's a lot of preparation involved, and careful observation. And if the game looks like crap, all that standing around is going to be really, really boring. Even when there is action, it tends to happen all in a burst before reverting back to stealthy creeping. Control is the other big point. Stealth controls need to feel fluid. When I play a stealth game, I want my character to move fluidly and with catlike grace as I climb about. When I tell him/her to do something, he/she needs to do EXACTLY what I say, because any slight error with the controls can leave me discovered and possibly dead. And then I'll be really annoyed, because I wasted 5 minutes stalking this guy and the controller failed to recognize my stealth kill command and hit him on the butt instead, alerting every jerk in a five-mile radius. So I guess in addition to good controls, stealth games should also have really good bug testers. Buggy stealth games are a pain to play.
In my opinion, my favorite stealth games tend to be the best controlling stealth games.