Gaunter O'Dimm. Unfortunately, to fully describe why, there'd be quite a few spoilers. Suffice to say, he's an incredibly charismatic, mysterious, and unnerving character.
CS:GO for the few days of the R8 being the most broken piece of bullshit I've seen in quite a while.
Seeing the Mouth of Mordremoth for the first time in Heart of Thorns. He's actually an easy fight, but I was part of one of the first groups to kill him, so nobody knew the mechanics yet and he's fucking gigantic.