Definitely Misty,she has great character and personality and in my opinon she had the best character interaction and chemistry with the group.She is the first girl,female protagonist and part of original thrio with who everything started who has quality and strenght.Her pokemon were amasing too,especially her Psyduck who was fantastic and many times maked me laugh.
Too bad that writers took her out,leaving her story unfinished(in many areas).Personally in my opinion the best thing would be to return her(it would be much better than another girly girl doing contests again),and even though chances are astronomically small,im still hoping for it.
May and Dawn are good characters who has their qualities and interesting points.They just didnt live up to my expectations like Misty did.
Standing in the background and mothering Togepi is not remotely interesting. Most of her Pokemon (aside from Psyduck) were entirely dull and uninteresting.
While its true,that writers should done more with her during Johto,she wasnt half as bad like some people say she was,and to be fair noone was threated greatly in this region.She still had great and memorable moments in this saga aswell(more than Brock received),and some of her pokemon aside from Psyduck, like Poliwhirl(later Politoed) and Corsola received a fair share of screentime aswell.
Also yes May and Dawn received more screentime,but its needed to take in consideration that during that time this show was mainly about Ash,so it isnt surprising why she didnt received so much screentime like they did.