Personally I think Echo actually has the most potential. She's a relatively new character to the MCU that we haven't seen much from, unlike Fury and Loki. And unlike Riri she's fully standalone and in no way a legacy title. She's also kind of in a sweet spot where she has had enough material in the comics to work with for a good introduction (mostly her storyline in David Mack's Daredevil Vol. 2 issues, but she's also been an Avenger and stuff), but she's also not been explored as much as she has potential meaning the show can do new things. So she's not a completely unknown character nobody cares about (like El Muerto), nor a character that has been used so much that there's little room for anything truly surprising (like Spider-Man or Wolverine). She's more in the middle like the GotG. She also has multiple aspects to her character that I think are underrepresented in the genre so far and will be interesting to explore in the show, with it both featuring Native American culture and having a deaf protagonist.
Now we'll have to see whether they actually deliver on that potential of course.