That first year of Shout's Corman line will forever be my favorite time for cult film viewing. I did enjoy PIRANHA very much. It's good campy fun. That said, I enjoyed most the other Corman BD's released around that time just a bit more.
GALAXY OF TERROR is the title I've been most anxious to revist since first seeing it in 2010.
How I'd rank them:
Humanoids from the Deep
Death Race 2000
Galaxy of Terror