I guess I'm always the odd-man out, but I personally liked the Mona Lisa.
Saw her twice. Once when I was very young and they had her in a very small room. I was not too impressed. The second time was more recently, after they moved her to a bigger room with better lighting, and I stayed about 20 minutes. It's hard to explain, but I find it hauntingly beautiful.
That being said, the most impressive thing at the Louvre is the Assyrian collection. The epic size of the lamassu is just breathtaking.