As the specialty of Brock, the first gym leader in Generation I, Rock-types have been a staple of the Pokémon series since the beginning. Many Rock Pokémon also have another typing, which can be helpful in battle, but only several pure Rock-types exist. A pure Rock-type is a Pokémon that only has the Rock typing. These range from mediocre to powerful.
There are several ways to evaluate strength, one of which is usefulness in the main series games. Serebii.net has information about each official Pokémon, and this information helps to measure viability. Though Arceus and Silvally can become pure Rock-types by holding certain items, they are not considered as such because they are pure Normal-types in their base forms. Although fully evolved pure Rock-types are not abundant, they can still be great in the games.
Stonjourner
As mighty as it may appear, Stonjourner has one unforgivable weakness: a terrible Special Defense stat. This stat barely exists and leaves the Pokémon incredibly vulnerable. Trainers can give it an item like a Focus Sash, one of the most useful items in a Pokémon battle, but it would only slightly help.
As for the things it has going for it, it has a great Defense stat, which can make it a capable option in matchups against Physical specialists. Paired with its solid HP stat, Stonjourner can be viable if it gets lucky with the enemies it faces. In these situations, its high Attack stat gives it the offensive power to compete, but a terrible Special Attack stat limits its capabilities to its Physical side. If trainers prepare for NPC battles ahead of time, Stonjourner can be great, but since this is usually not the case, it is often incredibly disappointing.
Sudowoodo
Sudowoodo was the first pure Rock-type to be introduced into the franchise. This Generation II creature has a lower base stat total than Stonjourner, but its more even stat spread makes it stronger. Sudowoodo has a great Defense stat, albeit a lower one than that of Stonjourner. It also has a much better, although not exceptional by any means, Special Defense stat. This means that Sudowoodo is vulnerable to Special attackers, but it can take more damage of this category than Stonjourner.
Sudowoodo is also more useful if it has the Ability Sturdy, which allows it to survive an attack that would have knocked it out in one hit, as long as it was at full health beforehand. It also has a relatively high Attack stat, letting it use the extra security from Sturdy to strike back with full force. However, its awful Speed guarantees that it will almost never move first in battle. Enemies can knock it out fairly easily, even if it does take an extra turn.
Rampardos
With an absurdly high Attack stat, Rampardos can be devastating if it moves faster than its opponents. The problem is that Rampardos rarely moves first due to its underwhelming Speed stat. Because of its lackluster defensive stats, it is one of the most underrated Pokémon from the Sinnoh region.
The pure Rock typing helps it slightly, as it does not have any extreme vulnerabilities, but it frequently gets knocked out in one hit anyway. If it survives this hit or moves faster than an enemy, though, Rampardos is a destructive force. Unless opponents have a very high Defense stat, Rampardos can often defeat them easily.
Lycanroc
This Pokémon is unique in that it has three different forms, and which form it takes depends on the time of day it evolves, as well as an additional condition for Dusk Form Lycanroc. All three have different stat spreads, but they only differ slightly for the most part. As such, they can all be considered at similar levels of power. The Midday and Dusk Forms have great Speed, while the Midnight Form's Speed stat is much lower but still decent.
All three forms have great Attack and access to some powerful Rock-type moves. The combination of Speed and Attack makes Lycanroc a dangerous opponent. While its defensive stats are not exceptional, they are passable. As a result, when Lycanroc rarely moves slower than enemies, it can usually withstand a strike or two before going down.
Gigalith
Like many Rock-types, Gigalith's Speed is atrocious. This can be terribly detrimental to many Pokémon, but Gigalith can take it with its capable defensive stats. Its Defense is very high, and its Special Defense is reasonable as well, albeit much lower. After withstanding a hit, Gigalith can strike back with great strength using its very high Attack.
Even if it is in a terrible matchup that would usually take it out immediately, Gigalith can stay in the battle if it has the Sturdy ability. It is one of the coolest Rock-type Pokémon in the series, and it has the power to go with its intimidating appearance.
Regirock
As the only Legendary Pokémon that has pure Rock typing in its base form, it is appropriate that Regirock is the strongest of this classification. While it fits in well with most of its peers because of its low Speed stat, Regirock has much better defensive stats. Its Defense is massive, and while its Special Defense is not quite as impressive, it is still quite high, even among many Pokémon outside of the pure Rock-type classification.
Though some of its peers have it beat when it comes to its Attack stat, Regirock's is still good. The added benefit is that Regirock can battle for much more time, letting it take care of opponents over a slightly longer period. This is different from others that rely on one powerful strike to defeat opponents. Regirock is more consistent and is an incredible pure Rock-type.