Made it through MONSTER LAIR the other day. It's a fun fusion of shooter and platformer, although it kind of overstays its welcome with so many levels with only small alterations in enemies and backgrounds each time. The core gameplay is solid, like all Westone games, and the pace is such that this makes for a fun, quick jaunt whenever the mood strikes. The bosses are big and interesting, and there's a real variety of them too. Having a two-player simultaneous option sets this one apart from most of its peers, and Westone was pretty forward thinking in the way the two players can share the screen without getting in each other's way (like being able to just stand on top of the other character to cover greater ground).
There's no question the music is best in the TG-CD version (even better than the arcade, really), but it's a toss up for graphics between the Genny and TG. The Genny has parallax scrolling in every level, sharper edging and better sprites for the main character (there's animated flight in the Genny and just a static sprite in the TG-CD). The TG has naturally better colors, animates smoother, and generally has bigger, better-looking bosses. I guess the main takeaway is that it's a good time whatever version you're playing!