Sadly the biggest warships I've managed to sink have been Destroyers; but man, the first one was one to remember:
I was in the SLOT in the Solomons and picked up a fast moving warship on sonar in heavy weather. I was sitting down at about 100ft just listening and waiting for the moon to come out. I think she was doing about 30kts but well within intercept range.
I lined up for my first Dick O'Kane method try (the only manual targeting technique I've managed to get right), and came up to periscope depth. Seas rough and the sky ablaze with the setting sun (wish I'd grabbed a few screenshots). I set my TDC and launch a full spread as the DD passes 340º.
A tense moment as I watch through the scope, waves regularly washing over it and obscuring my vision. And then, the awesome adrenaline rush and elation as 2 of the fish hit and break her back, sending her under.
Sadly I lost that boat the following morning to 8 or 9 destroyers which were steaming line astern in clear weather when I tried the same thing.