For me the order goes something like this:
1.May
2.Dawn
3.Iris
4.Misty
So that means May is my favourite. Now the reasons....
Personality: Not a great personality, she whines and loses confidence occasionally. Relied on others (especially Ash) a lot to begin with. Her personality is pretty much her biggest negative point.
Looks: In my opinion she's the best looking out of all main girls so far in Pokemon. Plus her outfit is coolest and not too short like Dawn/Misty.
Character development: The best character development among all girls. How she used to dislike pokemon in the beginning, how she got her first pokemon, How she started to like pokemon and became one of the top coordinators of Hoenn, how she got each and every pokemon of her team, her crush on Drew and how she became independent and started travelling alone in Johto almost everything associated with her was shown in the show. Now that's what you call a perfect character development. The only character in Pokemon who has a better character development than May is Ash. She also had a diversity in relationship with other characters. A sibling relationship with Max, Deep friendship and Mentor/student relationship with Ash, taunting rival with Harley/Drew, Mother-daughter with Caroline, Father-daughter with Norman.
Pokemon Team: With Blaziken, Blastoise(speculation), Venusaur, Glacion, Beautifly and Munch/Snorlax, her pokemon team is superior by far among all the main girls of the show. Only Ash, Paul, and Gary have a better pokemon team than May(among all main characters)...IMO
Skills: She finished among top 8 in Hoenn GF and top 4 in Kanto GF without the assistance of any pokemon of Ash or Brock. Results speak better than words. She's more skilled than Misty but Dawn's more skilled than May(based on what anime has tried to show us)
She's funny, neither too girly nor too tomboish, down to earth girl( You'd rarely see May praising her skills and beauty ). She had a nice chemistry with most of the characters of the show. She has wide range of emotions. She's generally really nice and kind, but has a tougher side for Max, Drew and Harley.
