My go to skin for Kodi is Eminence. It’s streets ahead of all other skins I’ve tried with regard to flexibility and sensible design decisions. Not perfect of course, but what is? The Kodi default Estuary is probably my second choice although it lacks some customisation options.
OSMC’s default skin is pleasant enough - although it too could do with some better customisation options. However it does have two major issues:
All cover art and posters are dimmed down to something like 50% brightness so only the currently selected item is at full brightness. This is a truly evil design decision! Surely the whole point of having cover art is to aid recognition when browsing long lists of music or movies. This is largely defeated by hiding all the art under a dark grey blanket … And, having spent countless hours hunting down the best art for my library I do actually want to see it!
Which brings me to the second issue - why does everything have to be so small? Not everyone has a 100" screen or sits three feet from it. Both fonts and cover art could be a lot bigger in many places without compromising the look and feel. This is a general problem with many Kodi skins.