I agree that Lionheart is an unfairly underrated game, probably due to the fact that it came out so late.
It has awesome graphics - especially the way the grass/tree "platforms" (or whatever you call them) blend from one colour to another. The graphics aren't as consistently beautiful as in Beast 1 though and some parts like the flying and riding could have been better. Beast 1 has some flat looking sprites, but for the time it came out, it blew everything else away - even arcades.
Lionheart gameplay is better than Beast, but I hate the flying/riding parts. And some parts (like the flooding castle) are so hard that I haven't been able to complete even in cheat mode.
Back in 1990, when waiting for Beast 2 to come out, I was expecting it to look something like Lionheart - with improvements in gameplay, fancy-but-different parallax scrolling and other fancy tricks (like water). I was horribly disappointed with Beast 2. So for me, Lionheart is more of a Beast sequel than either Beast 2 or 3.