This is hard to say. Obelisk's power is always defined at 4000, but his effect in the Anime allows you to sacrifice 2 monsters to make his attack Infinate. Slifer's ATK is determined by the number of cards in your hand x 1000. The Maximum is normally 6, but there are spells that allow you to hold more. Ra's attack is the combined attack of all the monters you sacrificed to summon him. He MUST be summoned with 3, so if you used Dark Magician(2500), Celtic Guardian(1400), and Red Eyes Black Dragon(2400) Ra's attack would be 6300.