Even though I like the 3 other couples as well, I like Philippe and Mathilde best. They're just perfect for one and other. You can see how much they love each other in little gestures, very sweet!
I also think that it's quite romantic -although it's very sad!!- that they really got close to each other when Mathilde's sister and grandmother died, and Philippe helped her through that difficult time.
You can easily see how much they respect and know each other, how they're a great team performing royal duties, both very committed to their country. And of course they're very nice and modern parents!