While the first two generations of the Pokemon series had relatively few legendary Pokemon in their Pokedex, with Red & Blue only having four and Gold & Silver having five, the Hoenn games, Ruby & Sapphire, decided to kick it up a notch and add eight legendary Pokemon to its games.
This wasn't the only increase in rare Pokemon, as there were also two mythical Pokemon found in Hoenn with Jirachi and Deoxys, which couldn't be obtained without triggering in-game events, whereas previous games only had one in both Mew and Celebi. Despite the increased number of legendaries, however, not all of them are worth using and the third generation games were something of a precursor to following titles having far too many legendaries for their own good.
Regice
While the Regis aren't necessarily bad, either each of them having very good defensive stats as well as a decently wide movepool at their disposal, Regice is easily the worst of the original trio. While it shares the same base stat total as Regirock and Registeel at 580, as well as one of the best special defense stats in the series at 200 and a very respectably 100 defense, it is definitely let down by its Ice-typing. Defensively, Ice-types are by far the worst, being weak to just about every common coverage move in the series such as Rock Slide, Close Combat, and Iron Head, however, it is unique for being one of the few Ice-types that can learn Electric-type attacks, though it also is one of the few Ice-types that can't learn Water-type attacks, severely letting it down in terms of move coverage.
Regirock
The next best Regi after Regice is Regirock, as the Rock-type is generally OK when it comes to defensive capabilities though normally lacks a wide movepool to keep offensive pressure on top of that. Regirock is no exception, though, like the other Regis, it can learn Electric-type attacks that help cover its Water-type weakness. Unlike Regice, Regirock can also learn Fire Punch, which it is able to make decent use of with its 100 base attack stat and covers its Grass and Steel-type weakness rather nicely. Regirock also has the opposite defensive stats to Regice, with 100 special defense and 200 defense, meaning it is more easily able to take common physical attacks that would otherwise threaten it, like Iron Head and Close Combat.
Registeel
The final Regi in the original trio, Registeel has by far the best defensive typing, especially in its original games where it not only resisted Steel, Dragon, Rock, Bug, Psychic, Flying, Poison, Ice, Grass, and Normal-type attacks, but also Ghost and Dark-types that were later taken away in generation six to help balance the type.
Registeel's offensive and defensive stats are also balanced between each variation, with 150 in each defense and 75 in each offensive stat. However, this does mean that it doesn't excel in any one area and is going to struggle to knock out Pokmon that aren't particularly weak, though it can cover its Fire-type weakness with Earthquake and Ground-type weakness with Ice Punch.
Latias
The first of the Eon Duo, Latias has fairly respectable stats across the board, with decent defense, HP, and speed to go along with its good special attack at 110 and great special defense at 130. Even without the Soul Dew held item, which boosts its special attack and speed by a whopping 50%, Latias is more than capable of both defending from threats as well as taking them out, with access to moves like Calm Mind, Dragon Dance, and Tailwind to ensure both it and its teammates can come out on top. While it is good, it falls slightly short of its counterpart due to its emphasis on defense rather than offense, requiring the use of stat-boosting moves to deal any significant damage.
Latios
The more offensive of the Eon Duo, like Latias, Latios has a base stat total of 600, the same as most legendary groups like the Legendary Beasts, as well as pseudo legendary Pokémon. Unlike Latias, however, Latios is much more geared towards offensive prowess than support and is capable of dealing devastating damage when given the chance.
Its signature move, Luster Purge, also has the added benefit of potentially reducing the target's special defense, meaning repeated use on the same Pokémon will do increasing amounts of damage. When paired with the Soul Dew, Latios becomes downright terrifying to face down and is capable of effortlessly sweeping through entire teams if given the chance.
Groudon
The legendary Pokémon who breaks the earth with every step and can force mountains to erupt just by existing, Groudon is a Pokémon that many should be glad doesn't exist in real life. With a base stat total of 670, there are few Pokémon out that that can match the raw strength of Groudon, not even taking into account its Primal form in generations six and seven. With its Primal Reversion, its stats skyrocket to 770, gaining increases in its attack, defense, and special attack from 150, 140, and 100 respectfully, to 180, 160, and 150. With these stats, it's next to impossible to defeat Groudon with a physical attack, even if it is super effective. Due to its Drought ability, it also reduces the damage taken by Water-type attacks while also giving it the ability to fire off Solar Beams without having to wait a turn. Unfortunately, the Sunny weather condition just isn't as good as the Rain condition, so it just misses out on being better than its counterpart.
Kyogre
As the cover legendary for both Sapphire and Alpha Saphhire, it should be no surprise that Kyogre is quite the powerful Pokémon. Like Groudon, its base stat total is 670 in its base form and 770 in its Primal form. Unlike Groudon, however, it gains stats in attack, special attack, and special defense, going from 100, 150, and 140 respectfully to 150, 180, and 160 after Primal Reversion. Just as Groudon can use Drought to power its Solar Beams, Kyogre can also use Drizzle to not only power its Water-type attacks, but also allow it to use Thunder without accuracy checks, something that any Water-type benefits from due to how they normally have a difficult time dealing with other Water-type Pokémon. However, as strong as Groudon and Kyogre are, they only stop fighting in the presence of one other Pokémon, who also happens to be the strongest legendary in Hoenn.
Rayquaza
Considering Rayquaza is the sole reason why Kyogre and Groudon stop fighting, it should be no surprise that it is not only the most powerful legendary in Hoenn, but also the most powerful Pokémon native to the region with a base stat total of 680. This Dragon/Flying-type not only has the Airlock ability, which neutralizes all weather conditions, but is the only Pokémon in the series who doesn't need a Mega Stone to Mega Evolve, instead only needing the Dragon Ascent move in its moveset. After Mega Evolving, Rayquaza's offensive stats in attack, special attack, and speed go from 150, 150, and 95 respectfully, to 180, 180, and 110. With the same attack stats as both Kyogre and Groudon's best defense stats, there is little that either Pokémon can do in the face of their referee.