[Fiverine]

Here are a few of mine.  Feel free to add yours!

When my opponent drinks a healing potion and my attack of opportunity is a critical hit

When I pass the spot the arch-rival of my previous PC died at

When an undead monster drinks a healing potion

The death animation of giant frogs

When an enemy mage casts magic missile and I've got shield or blur up

[Leyoz]

I guess that level up ping counts as a big pleasure.