Okay, not being intimately familiar with EITHER of these guys means I had to look ALL of this up.
First off...Fortnite has LORE?!
fortnite.fandom.com
fortnite.fandom.com
en.wikipedia.org
Okay, so...my impression of these characters is this:
Raz with the power of the Cube is that he's basically Aladdin if he got possessed by a Cosmic Cube.
Blackstar is He-Man, but he has a pet dragon instead of a tiger. The Power Star is basically exactly like the split Sword of Power, the villain even usually has the other half. The complete version apparently had enough power for the villain to rule over the planet before it got split.
Soooo....hmmm...I feel like Raz SHOULD have more power going by the LORE...but in the game itself he gets taken out by a rando with a gun (the player).
And a big problem is that I don't think we really have any idea what the upper limits of a complete Power Star are. In theory, Blackstar might even be able to share the power with Warlock the dragon and then the fight would be two against one.
I guess I'm going with Completed Power Star Blackstar and Warlock? I'll admit, I'm going in blind here.