I would contend that it is one of the most demanding sport in the world. Now, let me be clear, this isn't taking away from any of the other sports. They are all difficult in their own right, and I have a great deal of respect for any olympic athlete for the time and effort they put into the trade. However, if forced to compare them I would suggest that very few of them are comparable to wrestling. Unlike other sports, every single muscle is going 110%. You have to be so incredibly well-rounded. You can't just be fast, you can't just be strong, you can't just be incredibly well conditioned, you can't just be intelligent, you need all of it. It's a pure sport, and by that I mean it's one man against another. No teammates, no one to get your back if you falter, no tools or balls to mess around with... just a battle between two soldiers. Sports that might compare... other 'MMA' sports such as judo. Maybe (maybe) boxing. Both are sports which like wrestling pit one man against another, straight-up. Both feature weight cutting which adds tremendously to the difficulty and commitment required of the athletes. Doing something on a full stomach is one thing... doing it after not eating much for two days and working out 6 times a day is a completely different thing. There are many reasons behind my views... this really just touches the surface, but unless you've tried it for a good while you're likely not to understand. And that is ok.
Just my 2 cents. Well, maybe 3 cents.