Even after so many new girls its still Misty and ill explain why:
In my opinion she had the best chemistry with group,providing many entertaining and funny moments making this show more interesting.She is a great character with amazing personality,and her interactions with Ash and Brock were truly classic.Her dynamics with group,doze of witty comments,sarcastic gestures,eruptions of anger provided to this show and still does whenever she appears something,which is lacking in these days. In my eyes none of girls which came after her didn't managed to encapsulate emotions, drama and tension in such genuine and endearing way which Misty provided.
Nevertheless she is also very intelligent and resourceful as person being able to adapt to any situation(which case with st.Anne arc,taking care of Ash and Tracey during OI etc).
She was type of character who didn't had easy life having to fight hard for everything she accomplished encouraging others to believe in themselves and their dreams,providing big moral support helping Ash's growth as trainer along with having several notable achievements(like winning Princess festival,Seaking competition,being among top 8 in prestige water tournament,saving people lives or towns etc).
Another thing which stand out to me about her is that she was only main character so far who trained one type wanting to become best trainer with them,learn everything she can to become water pokemon master one day.Whole dream being unexplored and undefined added that mysterious quality to Misty allowing writers to have multiple directions through which they could take whole thing which unfortunately didn't happened.
For record: Im not the one who evaluates if character was good or not by in how many episodes he starred in or if he did contests, battled a lot etc, because someone value shouldn't be measured through how much he does, but by how valuable things he contributed to show were.
For example Misty may have did less than other girls did,but that doesn't make her inferior as character because its quality which counts not quantity. Misty served as coach and motivator in main cast pushing others to try their best(thanks to her Ash became better trainer with this not being only Brock merit).
She often brought encouragement to others playing role of classical heroine saving other lives and pokemon,providing moral support.
Had great personality,bringing healthy interactions and dynamic within group containing lot of humor, tension and chemistry often turning episodes which are on first look mediocre in enjoyable ones.
Likewise she got some notable growth herself too happening gradually. Like getting over complex of feeling less worthy compared to older sisters, becoming more mature and confident over course of original series with Togepi helping her in that growth by exploring more on different sides of character while keeping that quirky, hotheaded side of hers along with improving skills as trainer.
And while i know some people are not too much supportive of this idea i would like if Misty could return as main character being interesting to see her being developed more.Writers never developed her enough with lot of potential being left untapped.