Overrated:
100-400L IS. It's a convenient, versatile lens, but at 400 it's soft wide open, and since it's f5.6, it's frequently wide open. It's also a dust-pump, no matter what the deniers insist.
100mm f2.8 Macro. Maybe it was me, but wide open this lens seemed soft. Stopped down a bit it was good, but how far do you want to stop down a macro lens?
Sigma 50-500. Not a BAD lens, particularly for the price, but a very difficult lens to be good with. Heavy, front-heavy, hard to zoom, very slow.
Underrated:
Canon 400mm f5.6L. Just an incredible lens, sharp even wide open and very light and handy for the focal length.
Sigma 24-70 f2.8. Very sharp, with no vignetting even on a full frame camera. Yes, it's slow to focus in low light, but you can't have everything.
18-55 Kit lens. It's not built very well, but it is very capable of taking great pictures.