The Johto region didn't introduce the same number of new Pokemon that Generation I did, but it certainly had its fair share of exceptional Pokemon despite that.
While this list is primarily for use in Generation II itself, it will also focus on how useful these Pokemon have been afterward, all the way to the present-day Generation VIII.
Note: This list is subjective and reflects only the opinion of the writer.
Top 5 new Pokemon introduced in the Johto Pokedex
#5 - Skarmory
This metallic terror of the skies was one of the first Steel-type Pokemon introduced. With such a strong start, it's no wonder why Steel-types are still regarded as some of the premier defensive Pokemon to this day.
Skarmory has an insane 140 base defense. Along with resistance to most physical types in Generation II, nothing short of a super-effective move will be able to do any real damage to the sharp-taloned terror.
The moveset is noteworthy. With moves like Rest, Curse, and Drill Peck, Skarmory can dish out just as much damage as it takes.
While Skarmory may have fallen off in the most recent Generation, it upheld its role as an airborne tank long enough to still be considered a top Johto Pokemon.
#4 - Scizor
Scizor is another one of the new Steel-types added in Generation II and an evolution to a preexisting Pokemon, Scyther. Like Onix into Steelix, Scizor's evolutionary method was also new to Generation II, holding a particular item while (Metal Coat, in this and Steelix's case) while being traded.
While this method of evolution may be annoying, it was still a new and interesting concept. Scizor is an exceptional physical powerhouse capable of learning several useful setup moves like Agility and Swords Dance.
While Scizor's true power in battle won't be unlocked until later generations, it still has plenty of power in Generation II.
#3 - Umbreon
Umbreon and Espeon are the two eeveelutions added in Generation II. While not all eeveelutions have been the greatest successes, these two certainly were.
Umbreon is one of the best tanks in the entirety of Generation II, and Espeon is a premier attacker. Two sides of the same coin, both evolved via friendship from the same Pokemon based on time of day.
Friendship evolution and the day/night cycle were new mechanics added in Generation II, as was the Dark-type. This makes Umbreon, in particular, an essential new addition to the Pokemon franchise, as it represented all three.
Additionally, it's an extraordinary Pokemon in its own right makes Umbreon one of the top new Pokemon of the Johto region.
#2 - Tyranitar
Generation II's pseudo-legendary certainly lives up to its title. The hulking behemoth is one of only two pseudo-legendaries to date, not to be a Dragon-type. It's also a premier example of the new Dark-type, showing off just how useful it could be.
Tyranitar is a physical powerhouse with impenetrable defenses and enough special attacks to make special moves worth using. While it has a wide array of useful moves, it can make excellent use of Earthquake, Crunch, and Rock Slide.
#1 - Lugia
While both Gold and Silver's token legendaries are amazing Pokemon, the legendary has shone the most since its Johto debut is undoubtedly Lugia.
Lugia was already one of the best walls in the game in Generation II. It was arguably its weakest appearance in any game so far. It's difficult to injure this Pokemon seriously.
It can heal itself with Recover (and in later games, Roost). It can dish out serious damage with Psychic and Aeroblast. This Pokemon is an absolute menace. Without question, Lugia is the top new Pokemon from the Johto region.