Easily MGS.
Splinter Cell 1 & 2 suffer from heavy trial-and-error. They finally got the right idea with Chaos Theory, allowing the player more flexibility & the ability to actually fight back if you're caught. MGS1 and MGS2 feature trial-and-error as well to an extent, especially MGS2 when you enter areas without a radar. The thing is, those games are all faster & feature snappier gameplay/controls than the original Splinter Cells.
Let me clarify: If you fail in MGS2 you can reload at the start of the level in 10 seconds. There is always a faster & more efficient way to go through areas and avoid guards than just waiting for a patrol to pass by. The way Splinter Cell 1 & 2 work, however, you often have to trail an NPC or wait for a patrol to repeat this pattern. When you fail in Pandora Tomorrow, you have to wait for the same sequence of events to play out every single time, which is notably annoying in more scripted levels like LAX. You can mitigate this by savescumming, but at that point I'd just play a different game. Its no coincidence that one of the lowest points in MGS series is the MGS4 Act 3 escort mission which bears a strong resemblance to those early Splinter Cells. Doesn't hurt that MGS1 and MGS2 also have hundreds of extra missions.
MGS3 and Chaos Theory are just great games all around; not much to say on those two.