Worst Legendary/Mythical Pokémon?

Battle-wise, the worst (other than Cosmog and Cosmoem which are pre-evos) is probably Phione, It's stats are pretty bad, it's ability doesn't really help it much, and it's outclassed even in PU (the lowest usage-based tier)

 

Regigigas because it has the worst ability ever, slow start

 

Probably Type: Null because it's so hard to evolve

 

Merely the word "Regirock" is enough to strike fear into any sane person's heart. This thing has driven multiple of us Lake Valorians into insanity, me included.
- Pollo de Loco (thanks @Arod )