They're all good, but
HMS Pinafore is their earliest hit, and I always feel it is lacking something.
When I first heard hits from
The Pirates of Penzance at the age of six on my father's 78s, I was not familiar with Verdi. But a really good parody can be amusing even if you don't know what's being sent up. I only just made the connection between
Su del lido del sacro Nilo in
Aida and
Go ye, heros, go to glory in
Pirates.
The Mikado is their best work, although I know at least one G&S enthusiast round here who has got tired of it as a result of overexposure. But it is the one I would go for, if you ask me.