Robin and Marian
We don't see much of the actual Marian (thanks Zelena) and Robin together, but even from the little screen time they have as a couple, you could tell that their relationship was sweet. They truly loved each other and it was an example of a good couple (of thieves!). Even Rumple ended up having a soft spot for them when he saw that Marian was pregnant with baby Roland.
Regina and Daniel
Speaking of sweet, that was definitely Regina and Daniel. They're a great example of what young love looks like, and a forbidden-ish one at that. You can tell why Regina took his loss very hard. Does it excuse her becoming the Evil Queen? No. But Daniel was such a sweet soul that didn't deserve to die the way he did. And they were truly good together.
Henry and Violet
Another example of young love, Henry and Violet were so sweet too! I loved how they were awkward but cute, and Henry would show her everything Storybrooke had to offer in helping Violet assimilate to her new home. And even when they were in Camelot, she taught Henry how to ride a horse and get his heart racing. Honestly, these two had more chemistry than adult Henry and Cinderella.
Robin and Alice
If there's one good thing that came out of the seventh season of Once Upon a Time, it's Robin and Alice. While the two had a rocky first meeting, we got to see their love story play out in Enchanted Forest 2.0. And they couldn't stay separated for long in Hyperion Heights even if they were cursed. Robin was very protective of Alice, and the two really cared about each other. I like how as Alice was cursed and had "bad days," Robin stood by her. And the two get engaged by the end of the series. Yay!