Reading through this thread just made me realize that I haven't had a hucard played through a stereo in a very long time. The Duo is pretty much for CD games and the Express is for the hucard games these days.
The post about R Type reminded me of something funny in the early days of my playing. I bought a Duo with some CD games first, and it wasn't until a few months later that I bought my first Hucard, R-Type. Being a big fan of the series, naturally it was awesome. However it made the next few hucards I bought sound like total trash.
Personal favorites:
R-Type
Parodius (note-I have played the SNES/SFC version and although it has "superior" sound quality, I like the PCE versions of most of the tunes better)
Salamander
Pac Land - no, really. It only has one song, but I like it. With the music and the in game sounds playing, it sounds like a whole lot is going on, and it reminds me of an old arcade with all the machines turned up too loud.