Of all the old systems I still spend time playing (and still spend money on), the PC Engine probably is the one with the most picked-over game catalog.
What I mean by that is there aren't many "hidden gems" in its library. Very few of the lesser-known or oft-ignored HuCards and CDs released for the console are must-buy or must-try titles, in my opinion.
Still, I'd say the handful below fit that bill. So if you're looking for a new PC Engine game or two (or five) to pick up and play, you could do worse than try the following. (And if you're looking for five Japanese PlayStation games you've overlooked and need to play ASAP, erm, click the embedded link to the left.)
Gekisha Boy--OK, so this Irem-made side-scroller isn't exactly an unknown quantity. I'd definitely say it doesn't get as much love as it deserves, though. I mean, a game that looks and sounds as interesting as this one does really should have a higher profile. (Listen to an example of its soundtrack in this recent post.) Plus, Gekisha Boy's gameplay is unlike almost any other title--PC Engine or otherwise--I've experienced. For that reason alone, I'd highly recommend trying it if you have the means and the time.
Hany on the Road--This Arc-made and Face-published HuCard is far more overlooked than the one discussed above. Don't expect it to be quite the treasure Gekisha Boy is, however. In all honesty, Hany on the Road is an imperfect game. Although its gameplay (which recalls Capcom's SonSon) is unique, it could be argued that it's not exactly compelling. I don't agree with those critics, mind you, but even if I did I'd still suggest giving it a chance--especially if you've already experienced better-known PC Engine platformers like Mizubaku Daibouken, Parasol Stars and the PC Genjin titles--because it tries something different.
Obocchama-Kun--Namco released a whole lot of games (some arcade ports, some original efforts) for the PC Engine back in the day, and many of them are now fondly remembered classics. A few examples: Dragon Spirit, Galaga '88, Marchen Maze, Pac-Land, Pro Tennis World Court, Splatterhouse, The Tower of Druaga and Valkyrie no Densetsu. Obocchama-Kun isn't one of them. That surprises me, as I've always found it to be a nice (and silly) change of pace from the console's many other platformers. So, if you've played more than your fair share of Adventure Island and Bikkuriman World, check out Obocchama-Kun. (Bonus: a complete-in-box copy shouldn't cost you more than about $20 these days.)
Pop'n Magic--Everybody likes a good Bubble Bobble clone, right? That's what I've always thought, at least, but you wouldn't know it based on this game's shockingly low profile. Most who are aware of Pop'n Magic, though, love it. In fact, one swears it's better than the mighty Parasol Stars. I wouldn't go that far, but I agree that Pop'n Magic is a stellar experience in its own right. Its protagonists are every bit as cute as you would expect from the genre and its backing tunes are honest-to-goodness bops. Most importantly, though, Pop'n Magic's gameplay differs just enough from its competitors to seem like a breath of fresh air--even if you've worn out your Don Doko Don HuCard and Rainbow Islands CD.
Tricky--I've got to be honest here: I long ignored this Alfa System-developed and IGS-published puzzler because of its Japanese cover art, which I considered to be sub-par. (Still, it's miles better than the North American counterpart.) I also wasn't much of a fan of Tricky's in-game graphics. For whatever reason, though, I eventually shoved those negative opinions aside and put a few minutes into the game. That's all I needed to change my tune. Hell, after playing Tricky for a bit, I've even come to like its spritework, which is no doubt rough but also displays plenty of charm. If only I could finish a few more of its levels. (Seriously, most of them stump me to the point that I wonder if my brain has stopped functioning.)
Note: all of the screenshots included in this post were taken from the wonderful Video Game Den