Regina and Robin
Sigh. Outlaw Queen. I still haven't forgiven the writers for killing Robin off. The whole Evil Queen/Regina splits herself into two people storyline was so unnecessary. It was a good way of redeeming herself, but she was already on that road and it was being handled much better before that season 6 plot. Robin and Regina brought out the best in each other, and Robin had so much faith in her. For a long time she didn't have love, and he gave her that and more. They were a great couple that should have been endgame. Well, they are in my head at least.
Snow and Charming
The ultimate OTP and True Love pairing of Once Upon a Time is definitely Snow and Charming! These two prove how powerful their love is. They've made sacrifices for each other, but also supported one another no matter what. Right from the pilot, I was so invested in Snowing's story. And as it continued to play out, it got better and better. No matter the obstacles in their way, they never gave up on each other and held onto hope. It was a beautiful love story to see play out for six seasons.
Emma and Killian
In my completely biased opinion, the best couple on Once Upon a Time is definitely Emma and Killian. Aka Captain Swan! There's just something so special about this couple. Both of them have unfortunate similarities: grew up orphans, lost loved ones, and led mostly lonely lives. Emma's life started to become full of loved ones with Henry, her parents, and friends in Storybrooke.
And when Killian came along, he added to that. Killian wore his heart on his sleeve, and was careful with Emma's heart. Yes, they had their ups and downs. But both of them sacrificed for each other, took risks for the other, and proved to be a wonderful example of a healthy relationship. After the years of growing up alone, both of them deserved to be happy, and they got that!
All seven seasons of Once Upon a Time are streaming on Disney+ and Hulu.