The Defence form kicks but.
The Attack form has a Defence comparable to Blissey and a Special Defence to match. Nearly any Pokémon with Quick Attack or Extreme Speed could kill it.
The Normal form is just the Attack form with slightly better Defences, but worse Attack stats. Again sucks.
The Speed form has Speed and...that's it! Seriously, this guy sucks so bad that it actually got degraded to OU. It can't give a punch, it can't take a punch. It just sucks.
The Defence form is the only one who I would consider using. It is good at stalling out the opponent with Taunt, but still is nothing compared to Lugia. Although I don't like any of them, this is the one I like best.