Dragon Master Knight is one of the huge Fusion Monster cards from the animé/manga.  Simply put, it is mostly a novelty as playing it is incredibly hard: it is a Fusion of a Level 8 Ritual and another Fusion Monster composed of three copies of the same card.  If you did things “classically”, then that’d be at least three Spells and five Monster cards to bring this out (six counting Blue-Eyes Ultimate Dragon itself, though one might not since it won’t need to be drawn into or searched out).  Now, you do get a Light/Dragon with the highest printed ATK/DEF scores in the game (5000/5000) and an ATK boosting effect.  Unfortunately, this means next to nothing.  It is still vulnerable to Monster removal.  The conditions of the boost are such they are unlikely to be met even in casual duels, and even though this is a Dragon, using Dragon’s Mirror is probably not going to do much good since you want to use a Fusion substitute with this card.

 

Of course, unless Memory Crusher is a problem in your area, you could still risk running it.  After all, padding one’s Fusion deck can confuse an opponent.  So, as a Fusion it would be a 5/5 for all formats, but its actual usefulness is near non-existent, so it should really get a 1/5.