Who do you think Ash should marry when he grows up?

I agree with you on the Serena situation. They're meant for each other.

Should serena

I think Ash should marry all of his companions simultaneously. This practice is illegal in many, but not all, countries in real life. However, Pokémon is fictional so we are free to let our imagination run wild. After becoming a Pokémon Master and undoubtedly winning loads of prize money, Ash definitely has the means to support his many wives and to provide all of them a high standard of living. If he picks one girl to be his most favored wife, it would be Serena. Honorable mentions to Dawn, Misty, and May.

Non-exhaustive list of girls in Ash's h*rem:
Misty, May, Latias, Dawn, Cynthia, Lyra, Iris, Serena, Miette, Lillie, Lana, Chloe Cerise

This solution also sidesteps the numerous arguments among fans regarding who Ash should marry, otherwise known as shipping wars. In this situation, everyone can have their cake and eat it too.

WHY'S CYNTHIA, A GROWN ADULT IN AT LEAST HER MID TWENTIES ON THAT LIST!? Ash's 10 years old! In real life, he would still be in Primary school, while Cynthia is A GROWN ADULT who would have finished college already and got a job.

This is why I hate the topic of shipping in the anime side of the fandom, so many people being weird about literal 10/11 year olds.

This topic is quite divisive given the Shipping element involved. For those who started with the Anime at its inception like I did, we all have a soft spot for Misty but 4Kids admittedly pushed the two more than the Japanese Dub ever did. They've even teased Misty with other guys(notably in the Orange Islands arc) but deep down I feel she does have feelings for him like she did for Red in the Adventures Manga. Whether Ash has feelings for her is debatable.

May or Advanceshipping I never really understood. It felt like more of Mentor/Student relationship if anything and they heavily implied that Drew liked her so that to me kills that shipping before it leaves the dock. Only reason I think Advanceshipping got any legs is due to Adventures Manga where May admits her feelings to Brendan. Pearlshipping was more of a true friendship than anything with each one learning from the other. They did tease it with the HG/SS Episodes with Lyra but I don't think it has much steam given Kenny and Penguinshipping exists.

Iris or Negaishipping, just doesn't fly with me. Iris was their way of going back to the Gen 1 roots with an entirely new companion cast(no more Brock) and Ash catching lots of Pokémon(the most he had caught in a long time up to that point). They turned Ash into a rookie buffoon at times and they did Iris dirty by not having her get any Dragon Pokémon until ~90 Episodes into the series. Completely redeemed her in Journeys but I would have preferred more Dragon Trainer development during Gen 5 Anime.

I'll save the "obvious" one for last by skipping ahead to Alola. Three girls this time around means three too many. There wasn't really any chemistry between Ash and Mallow or Ash and Lana despite Lana's sisters teasing her on the subject. If anything, there was more Chemistry between Mallow and Lana given their childhood friendship. As for Ash and Lillie, that also didn't feel like it got anywhere. Ash certainly had a rivalry with her older brother Gladion but in terms of romance, I just didn't see it. Friendship certainly but nothing romantic in nature.

Then there's Gen 8 and Chloe. Chloe had nothing for Ash that Generation. In fact, she got majorly shafted in Gen 8 and that was one of my biggest gripes in that the focus of the show was purely on Ash or Goh for the majority of it. I mean yeah she got some screen time but not enough and Galar getting ignored for the bulk of it was also a major letdown.

That leaves Serena or the rightful Kalos Queen(screw Aria). The writers put in a childhood backstory for Ash and Serena. Serena clearly has a crush on Ash and its way more than any of the other female companions prior or since. Like with Misty though, it's questionable as to whether Ash has feelings for Serena, but he did sacrifice for Serena when it came to her goal, something that I don't really recall him doing for Misty back in Kanto, Orange Islands or even Johto. That said, Serena did something that no other female protagonist has done: kiss Ash on the lips. All previous kisses to Ash were on the cheek. This one was on the lips and has been verified. Whether that broke through Ash's Oblivious ability, we may never know but right now Serena has the early and decisive lead in my book.