IMO...
Looking
Beasts > Regis >= Birds > Mew Brothers
Beasts looks ok, they were very well designed and the clouds in their back is kinda cool.
Regis get the second place because they were complex and simple at the same time. Simple design, complex idea. Maybe, the best idea among the legendaries (Including the non-trios), just gave you the most creative winner reason...
I think I can exclude Regigigas from the looking part. That thing is ugly, but interesting. What´s the problem with that? There´re a lot of loved ugly Pokémon. *Tries not speak Char* ops...
Birds in 3rd, they are ok, but something on all makes them boring... I think they obvious for their type. Of course, Regis and Beasts (Excluding Suicune because people can think it´s Ice at the first sight) had also obvious types, but they are very diferent than we see in the normal Fire/Water/Electric/Ice/Rock/Steel Pokémon.
Mew brothers I like their look, but they´re the same excluding the colored head x_x
Creativity
Regis > Mew Brothers > Beasts > Birds
Like said before, Regis had the best concept IMO, I like the Braille thing they added in the game, it was very creative and interacting (I didn´t just go to Serebii.net and see what I had to do, to get all of them (Didn´t even know about SPPf that time too). I took from a random site all the braille meanings and translated every plate, XD Two days thinking what "Relicanth came first Wailord came last" or something like that, meant). It was my favorite Legend catch. Getting a little off topic, Ho-oh mazes were also nice.
Regigigas added new things for a normal Pokémon. Dunno, the clown looking XD
I think the lake thing from Mew brothers was nice, better than "reach a place and see 3 little dogs running". Might change my mind about this one BTW.
Beasts are ok in the creativity set. Not very good, but not very bad.
Birds are fixed positioned, people did nothing with them and nothing new was added to them, almost no story (if they have, almost sure it´s worse than the other 3 trios)... wonder why it took last creative...
Battling
Beasts > Regis = Birds
Mew brothers can get any place. Maybe they´ll be between Beasts and Regis.
Beasts are just beasts battling! Excluding Entei, the Bests can make a great comeback for your team.
For the Regis, Regice is the only common used, but the other two has a lot of potential depending on your team. Their ability to absorb weakness attacks is great, then in the end they can just.. EXPLODE XD I love this attack.
Birds... they´re ok and got a good boost with XD moves, if it wasn´t that, I´d consider they worse than the Regis.
I think I´m done.
At the poll, I voted Regis. Because it´s the only Trio with four members XD LOL