Splinter Cell should take it here. Monkey Island 2 is, in its own right, a solid point and click adventure, and in its heyday was a big deal. Even today it can stand up as something of a cult classic.
Splinter Cell, however, is a phenomenal action adventure title, in terms of graphical achievement and damn good gameplay. In 2002, if you wanted the definitive action adventure title, you went to Splinter Cell. It's given rise to the Splinter Cell series that still continues today in the stealth/action adventure genre.
At the end of the day, I think Splinter Cell trumps Monkey Island, for both gameplay and lasting impact.