Hmmm, if you're going to go by the quality of the heart events and just the pre-marriage life as that's the point of the romanceable NPC, I think that Emily wins. Her events are weird, out there and very entertaining though the clothing one is ehhh, imperfect. I think her 10 event is, by a very small margin, the best. That said, I haven't gotten all the bachelors to 10 yet so I may be missing out. If you're looking for which rNPC has the best arc, then there are a few who clearly make a strong U-turn but that leaves their earlier events being less engaging than some might prefer.
My very first play through I ended up pursuing Sebastian, solely due to the fact that name ‘Sebastian’ is one of my favourite names. I’ve always loved that name ever since I was little. I do like a bit of his character arc development.
However, I came to like Alex more, due to his later heart events (I think it’s primarily due to the fact that I can heavily relate, and so his backstory hits home pretty darn hard).
Sam I think is just entertaining and fun, and whilst I do enjoy his events, I still prefer pursuing Alex over Sam.
I have a few that I suppose I enjoy more than others. Abigail is the one I identify with the most. Sebastian is the perfect spouse for a farmer who's always busy and on the go, plus he's a gamer and he loves frogs. Haley really grew on me and I find her downright adorable. Elliot is so creative and poetic. Maru's into STEM and is a health care worker.
As an oldster, I just can't imagine marrying such kids like Sam, Alex, Haley or Abigail (though I like them very much). Elliott is a snob. I like dandies, but not really as a husband. Harvey is mature, okay, but there's just nothing attractive about him for me.
I relate mostly to Emily, Leah and Sebastian, had even married Emily and Sebastian, but the latter is still as immature as the other boys, and Emily was a bit of a disappointment in married life.
I really like all of them, even or especially Shane, but as friends, not as spouses, and Shane always seems kind of unkempt. I'm sure he doesn't smell good. Maru - yeah, she's okay.
But Gunther! He's mysterious, and he's damn good looking. I remember my frustration in my very first save when I realised that he's not a marriage candidate, not even an NPC you can interact with or befriend.