The safest place to sit on a plane in the event of a crash really depends on the type of crash. But there is one consistent safety issue to seating. Sitting within seven seats of an emergency exit will dramatically increase your chances of getting out of the plane quickly and safely. The closer you are to an emergency exit, the better off you are. This seems like common sense, but it has been backed up by a comprehensive study of air accidents.
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In a 2007 Popular Mechanics study, they found that it is safer to sit in the back of the aircraft. Key points from the study:

1. It examined every commercial jet crash in the United States, since 1971,that had both fatalities and survivors
2. In 11 of the 20 crashes, rear passengers clearly fared better.
3. Only five accidents favored those sitting forward.
4. Three were tossups, with no particular pattern of survival. In one case, seat positions could not be determined.
5. In seven of the 11 crashes favoring back-seaters, their advantage was striking.

This diagram shows the survival rates for various seats:
http://media.popularmechanics.com/images/aircrash-seat-illo-0807.gif

Personally, I've always felt safest in or very close to an exit row.  Thinking about plane crashes, the cabin would likely become hectic with people trying to get out and could fill withsmoke depending on the problem.  Being close to an exit row would increase the chances of getting out of the plane without a problem.

You can find out which seats are on an exit row (and which are the most comfortable) by reviewing the maps at SeatGuru: http://www.seatguru.com/

When Popular Mechanics reviewed the best places to sit, they concluded:
"A look at real-world crash
stats, however, suggests that the farther back you sit, the better your
odds of survival. Passengers near the tail of a plane are about 40
percent more likely to survive a crash than those in the first few rows
up front."

All of this depends on the type of airplane and your personal level of comfort.  If you're nervous about flying and are a realist, I suggest that you review this article: http://www.pbs.org/wgbh/nova/planecrash/risky.html ; it is well written and clearly quantifies the risk of flight. 

Personally, I feel a lot more comfortable flying than driving on the freeway and am happy to watch a movie and do some work while I slip across the country

I'd guess tail or in the middle near the emergency exit.

The tail because a forward impact might spare you from being potentially bludgeoned to death by passengers and chairs and any other stuff flung forward from the rear of the plane (wear your seatbelt of course to avoid being flung yourself).

The middle because there's an emergency door (for what it's worth, at least if the plane lands you'll be near the exit), and if the plane happens to break up in midair, there's a decent chance the fuselage would break in or near the middle (try bending a twig in half, it'll usually snap in the center, similar principle). In that case assuming the plane is still at an altitude with breathable air, there might be less chance of being hit by debris, enabling you to fall free of the plane and try to skydive into something nonlethal.

Depending on how and where the crash occurs, you may be dead either way. Perhaps both (low) altitude and (positive) attitude would help in such a situation.

on the roof then jump be for it hit's the ground .

The safest place would be to stand on the wing. From there if anything goes wrong, you can jump off. But remember to wear a parachute, unlike the tennis players in this photo.