Shadowjeweler Hanar because literally everything. That art with the Thanos vibe (especially golden). The effect, the stats. Absolutely nothing bad about it and I'm super keen to play it.
Second option would be Kargath Bladefist just for the sheer power play it gives when you slam down a 10/10, kill a minion AND gain 10 armor all on the same turn. Mix it with Sweeping Strikes and your opponent can basically kiss any chance of winning goodbye.
I believe Sweeping Strikes won’t work with Kargath Prime, since he attacks and kills only one minion, even if he kills three in total.
For my favorite legendary, I think Astromancer Solarian. I really like Reliquary of Souls too.