Hitman series, Splinter cell ruled until chaos theory,
Wait, are you saying Chaos Theory was the last one that was good, or the first one that a was bad? In my opinion, Chaos Theory is the best Splinter Cell in the series. Double Agent wasn't bad either, with core mechanics based heavily on Chaos Theory. Also, I think D.A. wasn't on par with Chaos Theory because of all it's bugs, when it worked, it worked well. What a lot of people don't know is that there's 2 completely different versions of Splinter Cell: Double Agent. One version was released on the PC, Xbox 360 and PS3, and the other was the Xbox, PlayStation 2, GameCube, and Wii version. The games were completely different, and supposedly the previous-gen game was a lot better, it was developed by the studio that did the original and Chaos Theory, while the next-gen version was done by the folks that did SC

T. Anyway, I love the Splinter Cell series, and I believe it redefined the stealth genre quite much.
Man, I remember when I played Metal Gear Solid back on the PS1, what an incredible game of it's time. As a matter of fact, I picked up Metal Gear Solid: Twin Snakes some time ago so I could play it on my PC (with Dolphin) and enjoy the game yet again with better graphics. Sons of Liberty wasn't bad either, I just hated the fact that the main game took place only on that damn platform. As far as MGS3 goes, I've played that recently when I purchased the MGS HD Collection for the 360, and I was well through when my Xbox died, so I never got to complete it, seeing as I was not buying a third console.
Someone also mentioned Tenchu, and yes, that game was quite good too, but I don't remember it that much though, and I think they did release a PS3 version of it, but it was garbage.
Anyway, I think my favorite series is Splinter Cell, with Chaos Theory being my favorite. I just got Blacklist, and I'm actually enjoying it. I'm going through it in Realistic, and I saw somewhere they have some mode that makes it like Chaos Theory, by removing the Sonar Goggles and whatnot, although I have yet to try that.