Dragonball GT had, up until this point, been somewhat lackluster. It was funny at times, but overall, was pretty bland, lacking both the brilliant action sequences and exercises in character development that made Dragonball Z such a big hit.
This episode changed that. The scene finally returns to Earth, so the rest of the ensemble is brought back into the main storyline. Baby is one of the most compelling villains in the Dragonball franchise - the reasons for his hatred of the Saiyans are understandable, and his possession abilities make him all the more dangerous. There is finally a sense of urgency and excitement that GT had been lacking. "Baby's Arrival" also marks the point where the action sequences improve tenfold, bringing it on par with some of the better episodes of Dragonball and Dragonball Z.
Oh, and Vegeta shaves his moustache. That alone should earn this episode a 9/10.