[sarcasm]
I'd say that the most overrated skill is hand reading. The most underrated skill is passing SBDs at the table under the radar.
[/sarcasm]

Seriously, I think that the most overrated skill is bluffing. While it's definitely a very important tool, it's something that popular culture seems to put a lot of emphasis on.

Some people play poker as if it's the only tool in the toolbox by using it every hand. Obviously, these aren't winning players. These people don't realize that to run a successful bluffs you have to have the goods a lot of the time...otherwise, why would your opponent fold with, say, bottom pair?