There is a few games with seemingly unique engines that weren't used anywhere else. What they has in common is the strong vibrato on the tone channel, which I think is underexplored area in modern engines.

Saboteur and Sigma 7 engines has one tone channel and noise drums, Sigma 7 also has phasing effects going on the tone channel.

Galaxy Force has two channels of tone and noise drums. The second channel sounds somewhat like ZX-10.

Another game that seemingly has the same weird and rare engine as Galaxy Force - Count Duckula. The same year, but seems that it has totally different authors.

Another unique sounding engine that does not seem to be used anywhere else is in the game Transmutter.