Hah.. those are good!
Recently I was sailing to explore a new island. It was night and started to storm. During the storm I sailed into an area with no less than FIVE leviathans in close proximity. They are still there awaiting a time I need more Chitin. That was pretty cool.
The first time you encounter a 2-star Troll, or worse, a 2-star Fuling Berserker get the adrenaline pumping too. I've played so long I'm sure there are many cool moments that have blurred over time. Getting creatures to fight each other is definitely fun though. I like to lure Lox and Fulings over to new Tarpits and have them fight it out with the Growths.