IDK, I think Diablo 2 was peak Blizzard. We had StarCraft and Warcraft 2, and imo World of Warcraft was kind of the sign of the end, at least when it seemed they would keep doubling down on expansions instead of new games. I thought StarCraft 2 was just alright (bought Wings of Liberty on launch), and I didn’t bother with Diablo 3 due to it being always online.
So for me, peak Blizzard was around 2001. Granted, I never played Warcraft 3 (was just too different from the earlier Warcraft games), nor did I play World of Warcraft (didn’t have stable Internet, stable income, or stable time), so maybe the peak should be pushed out a few years.