2. Venus Terzo
Appeared in: X-Men: Evolution
Jean Grey is a pretty badass character and the reason I love X-Men: Evolution is that it actually recognizes that. Yes, the character has limitless powers that have the potential to overwhelm her, but she isn't portrayed as a damsel in distress in the way that the previous animated series unfortunately portrayed her to be.
Venus Terzo voiced Jean in X-Men: Evolution and she truly understood the assignment. The show revolved around teenage versions of the X-Men and this actually complemented Jean's story because it allowed us all to see her grow into her powers. There are similarities to the X-Men prequel movies in that we get to see a much younger Jean learn more about her powers, but this was a superior adaptation because she still felt inherently like Jean Grey; and it gave Terzo much more of an opportunity to shine.
As she's younger, she's powerful without being overpowered, but never too conveniently weak for story purposes. She grows into her powers, learning more about herself and Terzo does a wonderful job of capturing that, highlighting it with an even more confident performance in the later seasons.
As far as animated Jeans go, X-Men: Evolution is the best, and Terzo's performance is a major reason for that.