Rosalina's trailer is among my favourites not only because of the character, but because the animation was beautiful and it was objectively the most surprising trailer, in the sense that it was the only trailer we got where we didn't actually know it was about Smash Bros until the tagline came (the original showing of it in the Direct didn't have the flaming Smash Bros logo at the start), and we had no reason to expect a Smash Bros announcement in that Direct. I also loved the interaction between Kirby and Luma, which seems to have carried over into the game's opening reel and other material. In a way, Rosalina set the bar for me in terms of trailer presentation, and it set the bar for all of us in that we all started expecting trailers in every subsequent Direct. Not only that, but this trailer got me excited for non Smash things, such as other Nintendo characters in Mario Kart 8 - which is actually coming true (STILL WAITING ON THE PINK PUFFBALL HIMSELF THOUGH, NINTENDO). And of course, Gusty Garden Galaxy is always a beautiful song to have in a trailer.
Also, in terms of actually showing up, Rosalina is still one of the most surprising newcomers, and went against Gematsu right from the get go, which I liked since too many people used it as a basis for speculation. Even though die-hard Gematsu believers never believed it, Rosalina's reveal was earth shattering for the speculation community. Hence, her reveal may just be one of my favourites, if not my absolute favourite. Even without the fact it was announcing my most wanted character and would be main, it still MEANT the most to me, as a follower of the game.
Favourtie tagline is "Pac-Man Hungers For Battle!", though. Ironically though, I feel his reveal is my least favourite. The trailer isn't bad, but considering WHO it was announcing, it was VERY lackluster.