According to d4race.com, Twitch streamer cArn is the first player to reach Level 100 on Hardcore in Diablo 4! cArn is part of a 4-player party, consisting of Zizaran, Steelmage, and nugiyen. We do not know if these rankings are reliable, so we hope to hear official confirmation from Blizzard soon.
cArn on Twitch Barbarian Whirlwind Guide
The build he was running is an iteration of the Whirlwind Barbarian that relied on high Battle Shouts and uptime.