I think they are all great, and Conviction is awesome too but it is a little bit different to include that, as that is like the second generation of Splinter Cell games, if you get me. But for the old games (Splinter Cell 1 to Chaos Theory and even Essentials), I'd say this was the best one. I really enjoy #1 as well, but it lacks some actions such as opening doors with a body on your shoulders, which was a pain if you needed to hide it as the doors close. But it was the first one and it's still an amazing game! but PT is probably my favourite :).
I have yet to play this game. Its on my "playlist". I just haven't gotten down with version to buy... xbox, pc, or ps2. I played the original Splinter Cell on the Xbox so I might go that route... but I find myself playing games on the PC more now.