Cresselia is my favorite Legend because she was a gift from my best friend and pair, TheGr8. He surprised me by nicknaming the Cress after my nick on TC, EV. It's my absolute favorite mon ever cause he worked so hard to RNG it and then he went and NN it EV<3 after me.
Also you picked Dialga. May I asked why you picked such a powerful Dragon? I personally prefer Palkia because he just looks like a guy who can lift 50K lbs of steel
Not true, some of the Frontier Brains in the Battle Frontier also had Legendary Pokémon, though they weren't part of the main storyline, so I don't know if they count. Either way, yes, I do think more trainers should have legendary Pokémon. It'd be great to increase the difficulty in the game and give us more of a challenge, which is why it's often seen in hacks, and after this thing with N, I'm pretty sure we'll get to see it again. It seems there's being added more and more post-game stuff for every new game, so unless it's a rematch, I don't think it should be the champion that has one. I'd much rather see it for someone who should pose as the final challenge, just like Red in G/S/C and Steven in Emerald, so yeah.
I'm not sure how many of played Pokemon Stadium (the first one), but I remember a time in the Battle Dome the 2nd set of battles, the final trainer had a Mew. Mew literally gave me the most difficult challenge in that game back when I was young. I remember the day, the moment, after I defeated Mew (I had to beat Jolten afterward, thank goodness I had Golem's dig!), I was jumping up & down in excitement. It sure feels like a milestone to beat Mew back then lol