Best:
They weren't officially dating yet but Marcie bringing Kingston along to the pokemon she volunteers at (aka a good excuse to see him play with baby pokemon and see if he's father material) it was Marcie sharing something more personal/meaningful with Kingston and because she can be quite guarded with people the sign of trust in letting him into this side of her also meant a lot to him. It was that and seeing Marcie, sweaty and messy-haired in an old shirt sweeping up bits of poke-kibble from the floor that first made K seriously consider "oh I am in love" because it was deeper than the physical intimacy or attraction he'd experienced up to that point with her (and in past relationships with other people.)

That or after they'd been dating for a while and considering moving in together, they went to a department store and window shopped for potential pieces of furniture they needed to pick up, but it turned into them trying to describe an ideal home with each other. When Marcie leaned over a baby crib and teasingly asked if they'd have space for it she was surprised when Kingston answered 100% seriously that they could use the larger spare room for a nursery instead of a home office. Having that affirmation in knowing they have the same priorities in wanting to make a family together made Marcie treasure that memory since she's always wanted a normal family of her own and she really felt like she could see a future with Kingston.

I haven't even gotten to the worst dates RIP me

Uhhhh worst date for Kingston: having to sit through a very awkward arranged marriage meeting, made more awkward because the other party caught him trying to ghost the meeting even before it started.