Venti and Diluc are by far the most wanted and popular.
Keqing (Kokusei) is also very popular and in-demand. I think she’s bad and weak, though.
Right now the most powerful character in the game IMO is the new 5-star, Ganyu.
The upcoming 5-Star, Xiao, is also highly in-demand and a lot of people are scrambling to get him.