The triangle and the one on one are my favorites. But if I had to choose. I’ll choose triangle because the one on one seems to wishy washy. Nothing concrete. While a triangle has a dedicated determination strive. At least that’s how I see it
My preference is "protagonist x retirement fund", so I picked harem because the more the better, right?
Out of the given options I guess would say one to one. But the context of the romance makes a bigger difference to me than the number of characters involved. For example there is a pretty huge difference between a story where the romance starts from the beginning of the story or one where it only starts halfway into it. And in the latter case there can be a big difference if the romantic interest is an already established character before the romance begins or one that is introduced late in the story. In addition to that you also can have stories where the main character starts out in one relationship, falls out of love partway through the story and then begins a new relationship, all without involving a love triangle, and both relationships being treated as having been proper romantic plotlines.