Splinter Cell is definitely up there, but the Next-Gen version of DA was a flop. The Thief series was amazing. Assassin's Creed as well, but it's still in its infancy.
Above all of this though, is Prince of Persia, starting with Sands of Time. Now, I'm talking about the proper PoP series here, which doesn't include that cel-shaded monstrosity.
Assassin's Creed II and Prince of Persia: The Sands of Time are among my top five games.
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assassins creed is awsome, really cant wait for brotherhood to come out, luv the spinter cell too, was a little disappointed with the latest one tho, the story mode was just a bit short but apart from that no complaints