And some of the high ground is just plain wrong - some out by 1000 feet
Do you have an example? I know that Jeppesen terrain data is routinely out by anything up to a few thousand feet and mostly in the
unsafe direction (open up Flitestar for an easy way to see this) but the Garmin
496 has elevation data which, according to Garmin, is taken from the
SRTM radar imagery and this data is accurate to 100ft in the USA and 300ft outside. It has been found to be uncannily accurate in various locations, with mountaintop heights being spot on (and having obviously been manually fixed-up). I have just installed a G496 for use as a TAWS device - it's cheap at 10x the price, literally, just for this. The same data is also available for the GNSx30.
Back to the subject, I don't think there is a clear contender. Based purely on look and feel, the Avmap EKP IV would easily win IMHO. But the support is terrible and it will probably die in the market as the distributors gradually drop it.
The Skymap 3C is out of date by about 5 years, although it does everything it says on the tin. I fly behind a KMD550 MFD which is basically the same technology, and the display is OK. Then again, I have a separate GPS which drives the MFD (KLN94) and that takes care of the user-input interface, which on the Skymaps is horrid. I used to load flight plans from Navbox into my Skymap 2 and avoided touching it after that. There are serious defects in the elevation data there too, with the terrain colouring, and spot heights, being way off.
That leaves Garmin, Lowrance, etc and it's down to preference. I have little experience with these.