Ok, now let's get into sharpness. This is one of the most commonly cocked up settings, with most people mistakenly believing that always having as much sharpness as possible is necessarily a good thing. Actually, over-sharpening a picture just adds noise and edge ghosting. All you really want to achieve for the best picture quality is as close a relationship between a source and your screen as possible, as this takes more image processing out of the equation.
One of the first things you should do with image sharpness in mind if you have a Full HD TV is turn off any overscanning your TV may be applying when you're watching HD sources. Taking overscanning off allows a 1,920 x 1,080 source signal to map precisely to the 1,920 x 1,080 pixel resolution of your TV.
If you don't do this, then the TV will be applying processing to the picture to expand it so that the edges are pushed off the screen - and as you can imagine, wherever this sort of processing is taking place, there's potential for the picture to be softened or otherwise messed up.
The option to turn off overscanning is usually - though not always - found in the aspect ratio section of your TV's menus, and is confusingly called any number of different things depending on your brand of TV; Just Scan, Screen Fit, Exact scan, etc… The only good news is that even if it doesn't use an obvious name, it can usually be spotted by a process of elimination as the mode that doesn't just refer to an aspect ratio, such a 14:9, 16:9 etc.
Turning to the sharpness adjuster found on most TVs, the first problem you've got is that you don't know what default values a manufacturer has used. Some manufacturers ship TVs with sharpness set to ‘zero', so that nothing is being added to the picture. But others add a little. Also, some TVs will let you reduce sharpness below the ‘zero' level if you want to soften the picture to, perhaps, reduce grain noise in a source.
If you're trying to figure the optimum sharpness setting out for yourself, pause an HD image, and focus on a bright edge in it. Then, as you move the sharpness adjuster up and down, look for the point where the edge starts to look ‘stressed' - specifically, where a sort of halo starts to appear around it. As soon as you see this, nudge the sharpness back down one step so that, hopefully, the ‘ringing' noise disappears.
If your TV has an Edge Enhancement (or something similar) mode, it's very likely that activating it will cause edge haloing to kick in dramatically, so such tools should almost always be turned off.