Number 30 is the rank 3 with the highest attack in the game. In addition, Number 30 has the highest attack of all rank 4 xyz monsters and below tied with Number 85: Crazy Box.
Number 30 has three drawbacks. First, you cannot special summon any monsters while you control thise face-up card. Second, you will take 2000 LP if you don't detach an xyz material from Number 30 during every of you Standby Phases, and lastly, you cannot declare an attack with number 30 while it has no xyz materials.
Despite all of those inconviniences, this card is really good and that's because he has such a huge attack which is guaranteed to destroy almost any of your opponent monsters that can be destroyed by battle. Of course using number 30 against an opponent who plays defensively is suicide because he will just wait until Number 30 deals damage enough or even kills you as you are lockdowned by your own monster so be careful.