There are 3 quality Ash's main Love-interest need to have--
--Outstanding Beauty
--Dynamic Interaction
--A powerful connection with main plot of anime
"Outstanding Beauty" is most important thing!
Even if there is a girl with dynamic interaction and close friendship with Ash , nobody will support her if she's not cute at all .
Example : Angie , She is the closest female friend Ash ever got but nobody support her because she's not cute at all .
If The girl has outstanding Beauty then she practically need nothing else , Specially if the protagonist has a crush on her !
Unfortunately , Serena's Beauty isn't outstanding , Specially after she cut her hair ! Majority of member voted Dawn to be prettier then Serena.
The Girl with the most Outstanding & Unique Beauty is "May" ! Unfortunately , She's the only girl Ash can never have . The only girl that can match "May''s Beauty is "Rosa".
However , The Protagonist doesn't always end up with prettiest girl ! In that case , The Girl need to have close and Dynamic chemistry with one another.
Furthermore , Her Romance with the Protagonist need to play an very important part in the main plot of the Anime.
Pokemon focus on 2 main story in every series , One is "Ash Journey to be Pokemon Master" another is "Villainous ark" ! Some series has Side-quest for traveling Companion (AG ,DP) and some series doesn't (OS ,BW).
So ,
The Main pairing need a girl who play an important role in "Pokemon League" & Villainous ark ! A Very good example is Contestshipping Where May & Drew are rival therefore Drew plays the role as May's Love-interest and Her Enemy/obstacle at the same time.
Serena's Chemistry with Ash SUCKS ! Even Iris had better chemistry with Ash .
Plus , Just because she had some conversation with Ash before Viola's Gymbattle and he sometime look at her during gym battle doesn't mean she really play a believable role in Ash becoming Pokemon Master.
Serena's main Advantage was Her "Beauty" which she lost after cutting her hair.
So , Currently the Best Love-Interest for Ash would be a Female rival because she will be the lover & enemy/obstacle of Ash's dream therefore their Romantic development will play an important part of the series! Unless Writer create a "Mysterious Girl" who trying to defeat an Evil Team .
Ash pairings never really appealed to me, I'm more of a May x Drew fan. It had everything I hoped for in an anime like Pokemon.
Its because May & Drew has all the element of a Romantic Comedy !!!