Unown Crest: boosts special attack, and changes Unown's type to match its Hidden Power
I always felt bad for this thing honestly. It should at least be able to do one thing.
Shuckle Crest: Trick room at start of turn, swaps Attack and Defense
Because screw balance, memz are better.
Haxorus Crest: Adds the Steel type and applies Dragon Dance boosts
I just like this guy and was disappointed that it never got a mega evolution,
Marowak Crest: Switches base Attack and Defense, and allows Bone moves to hit Flying type Pokemon.
How are birds immune to a thrown bone attack? HOW!?
I don't know if Crests for legendaries are allowed, but:
Regice Crest: This Pokemon resists fire and cannot be burned.
In line with its pokedex entry where it will not melt.
Regirock Crest: This pokemon regains HP from Rock type attacks and Sandstorm
Also from the pokedex, where it repairs itself with loose stones.
Registeel Crest: Resistances are 4x, but weaknesses are also increased
Because I didn't know what to do for this one. The pokedex made it sound indestructible.