Twisted Tales of Spike McFang was really fun when it came out, but I haven't played it again since then. My memory tells me it's still fun.

It's not expensive, but it is pretty hard to find... I found a copy a while ago but it still hasn't made it into my system.
Pocky and Rocky 1&2 are both fantastic. I love em. When I started buying SNES games again those were among my first re-acquisitions.
Super Bonk 1 is excellent. Super Bonk 2 (no US release) has really weird control, so don't be taken in by the name alone.
How about Cameltry? I love that game! Marble maze games are fun~
Also, while it's not "underrated," if you like Prince of Persia, the SNES one is the best version of all IMHO. Probably has something to do with the fact that Konami made it. It's under-recognized in a way, as most people don't realize how different the various versions of that game are.
About Pac in Time: Do you know if Pac in Time for the SFC is different from the PC version? Yeah, I know this is a long shot... but it was released for the PC in English, and that's where I played it. Yeah, it kicks ass.