Gen 2 trio. I love their designs, exactly my idea "lesser legendaries". I don't know about the current situation, but Raikou and Suicune were also excellent battlers. Entei may not be as good, but I think he's awesome designwise.
The bird trio is of course great as well, they're classic and relatively simple, which is better to me than many of the slightly too weird designs of some of the more recent legends.
The Regis are a good concept imo and definitely convey an aura of mystery, but Regice is the only one I truly like.
The pixies are absolutely the worst of all the trios imo. Even the genies in Gen 5 at least have some different types among them, but the pixies are all Psychic, are boring designwise and Mesprit seems completely obsolete. I do like Azelf though, because of colouring and stat distribution, but I could have easily done without the other two.
The sword trio is actually quite decent. I really like Virizion's design and he's my favorite of the three. Terrakion is very competent and, well, designwise is a bit off here and there (he's too goofy), but I do like him. I don't particularly care for Cobalion though, but I don't dislike him either.