The best image will come by sending the native resolution of your plasma to your plasma. This is really only possible by using a PC and sending computer video to the display - and is not video so it's a silly way to do things. I mean, if your plasma is 1365x768 then it is not 1080i or 720p - it is 768p which is not a HD format... so all HD formats have to be scaled. But, PCs can put out 768p (video card dependant) native and put a pixel for pixel perfect image on your screen.
Now, what will provide the best image on your screen?
HDTV - that is, HD cable with an HD broadcast, like INHD, will provide the best image to your display.
Then it is up to your display to show it as nice as possible.
DVD, digital cable (non-HD), analog cable, off-air (non-HD), and video tape are all sub-par in quality compared to HDTV.
You can use off-air HD, cable HD, or DirecTV/Zoom/etc. satellite service HD. They are all the top of the line as far as displays go.
But, just because you are providing your set with the highest quality image does not mean that your set is actually going to show it the best possible way. You will want to adjust your set to display what you are feeding it as nice as possible. Unfortunately, some sets just do not have the capability to display video as nice as you would like them to.
CRT televisions (old and heavy) are still the best for displaying video, but really are going out the door now, but newer plasmas like your Panasonic should still provide a killer image with good contrast and colors. You will want to try using a display setup DVD to maximize the potential image on your screen, but you want to be careful to make sure you set it up with every input you are going to use individually.
HDTV MUST be sent over component cables or over DVI/HDMI cables. S-Video and composite cables do not carry HDTV... COAX from a cable box does not carry HDTV from the box to your plasma. Even if you see an HD channel, if you aren't on a component input or DVI/HDMI input, you are not seeing HD.
Hope that helps answer your question... You really want to make sure you have both a good HD source and are sending it to a properly calibrated display. The source is critical, but once you have a good HD source no matter what form of true HD it is (cable/sat/air) it will provide an ideal image that your display needs to deal with in tandem.
If after a lot of work you aren't happy, then you can bring an ISF technician in to calibrate your set. I would think an ISF approved tech would have a HD VCR with them they could hook up to your display so they can do actual HD calibration correctly.