wolfsrain: Alpha Protocol can be played stealth. With the right skills, your enemies will sleep like there is no tomorrow. But again, AP can be played however you want, just like Deus Ex.
Hitman series, on the other hand, are a must play. Thief series, too.
The original Splinter Cell is, definitely the best in the series. At least that's my opinion. The newer SC are simplifying the gameplay for the new generations.
I heard really mixed reviews on Alpha Protocol. I never actually played it because of that, although it did look cool.
I think Chaos Theory is the best of the series, although I haven't played Blacklist yet. Chaos Theory to me is where everything just came together beautifully. Adding the knife gave you a little bit more room to be violent and headstrong but it still remained incredibly important to remain as stealthy as possible. The shotgun attachment helped slightly when you were detected but the alarms kind of voided the usefulness of having equipment to kill openly.