Since there has been lot of lively and interesting discussion going over here i decided to come in and say my 2 cents over Ash and Misty relationship as well.
Now ill admit im not much of shipper of any sort, but looking back based on hints and amount of tension, strong interchangeable interactions and emotion Misty and Ash seem like most compatible pair so far.
Both have similarity in sense of being persistent, sometime stubborn and hotheaded(despite maturing Misty still has plenty of fire and brash side as noticed in interactions with sisters, when dealing with Hanson etc).
Had healthy and funny to watch friendly rivalry existing between them trying to outplay each other through various spats and jokes. And took battling between themselves seriously wanting to defend their reputation as trainer and prove to be better than another. Battle in Cerulean, for Totodile, in Whirl Cup or when entering races come to mind.
In alignment to that there was also present lot of care, personal understanding and motivation existing in their interactions. With Misty raising up his spirit whenever he was depressed, or let down having many moments of laugh, talk about their desires or fun, but annoyance too.
Misty often gave Ash moral support and encouragement to train instead of fooling around, or not to get too ridged preventing him from falling for TR tricks and imposters traps not to make fool of himself, motivating him to keep trying. Accompanied with lot of kindness and care coming from her side, being backbone on which Ash can rely. Something Ash himself started to appreciate showing lot of concern whenever Misty was separated from him and Brock or gratitude for curing him when sick, his life getting saved or standing up for him against others. I recall he also cheered for her as well when battling others, entering competitions and pursuing her quest of becoming water master.
Relationship between Ash and Misty felt naturall packed with tension, conflict and endearing to watch atmosphere having genuine, spontaneous and affluent characterization going between them from start to very end when their life paths got separated.
With Misty selfquestioning of how she feels toward it, blatant blushing and jealousy, along with feeling embarassed when accused by others of there existing spark from her side for Ash trying to hide it making it obvious at times how she feels something more than just friendship toward said character.
I wouldn't say it was so much Misty character fizzling out, but more starting to grow as person going through character development. She started to be less violent and impulsive at end of Kanto gradually beginning to change which reflected on relationship with Ash and way it played out.
Her character began to mature learning how to better cope with responsibility , become more open toward others not being so defensive and impatient with time growing in more compassionate and levelheaded character(allowing to get over Gyarados fear,cope with responsibillity saving gym, gaining sister respect and become more sure in herself) . Which came to expression further in chronicles and appearances still being sassy, headstrong and cynical, but also more playful, serene and prideful in her own abilities.
And i noticed just after rewatching some of episodes from Johto how much Misty friendship and bond with Ash really grew.
There was still arguing between Ash and Misty(such as for Totodile, when they used to get lost in forest because of him, when he was carefree delaying to deliver GS ball to Kurt, when she was teased over being Gyarados in horoscope etc), but violent aspect stopped being present in such straightforward manner ending replaced with more endearing, lighthearted atmosphere scolding each other . In form of comic scenes when getting on each other nerves like when Ash refused to try her soup,plenty of joking and sarcasm coming from Misty side, moments of annoyance when she bragged over being better trainer, Ash inciting remarks over she and other pokemon like Snubull or Houndoom being similar because of stubborness , laugh when talking about their dreams or embarrasing themselves in front of others.
Initial misunderstanding and intolerance during Kanto days was replaced with mutual understanding and lot of moral support coming from her side comforting him over loses, when leaving behind his pokemon like Charizard or when feeling let down growing to appreciate and cherish time spent with Ash with chemistry and interactions taking step forward evolving. While still being straightforward, feisty and grumpy scolding Ash, yelling or joking but in more grown up, matured way. I noticed this when Misty claimed for Ash and Brock to be her family she never had and biggest cullmination happened when having to depart from Ash. Being visibly upset and heartbroken over having to stop traveling, but even more over Ash reckless reaction like all those moments and time they spent together made it seem like he took lightly.
Accompanied with granted more subdued but still visible presence of affection and interest Misty holded for Ash coming out at times on surface. Aside from Macy scene at Johto league when she was all fired up and defensive over other girl approaching Ash, i recall her character getting jealoused few times at others as well like nurse Joy when commenting on how cute he is misunderstanding how she refered to Pikachu rather than Ash itself. When teaching Brock how to approach girls when Temaku wanted to marry him saying something along the lines" its alot harder to love someone who doesn't return this feelings back" heavily blushing when he asked her how she could know that.
Even until very end it seemed to me like that spark and Misty romantic interest never faded away if hint from chronicles "A Date with Delcatty" was something to go by declining Georgio offer saying embarassed how there is someone else(dub altered dialogue, but this is original line from Japan version).