Amongst all the weird evolution methods introduced with every generation of Pokemon, one method that has stood strong is evolution stones.
Every generation has introduced a few Pokemon that are assigned to one of the 10 evolutions: Water, Fire, Leaf, Thunder, Moon, Sun, Dusk, Dawn, Shiny, and Ice. They can only evolve with their respective stones, and while using items to evolve Pokemon can be a bit of a pain sometimes, it's usually worth it. These Pokemon have proven that they are worthy of being given the stones they need to evolve.
15 Ludicolo (Water)
Generation III introduced the Lotad line, which finishes with Ludicolo upon giving Lombre the Water Stone. The first Pokemon to carry two of the starter types certainly makes that an asset, as Water/Grass has proven to be a very underrated defensive type.
While its stats aren't great, it has access to a hidden ability, Swift Swim, that boosts its Speed in the rain. Given enough set-up, you could make Ludicolo a very competent rain sweeper.
14 Vileplume (Leaf) & Bellossom (Sun)
These two Pokemon evolve from the Oddish and Gloom line, and both do so through stones. The Grass/Poison Vileplume evolves from the Leaf Stone, whereas the pure Grass-type Bellossom evolves from the Sun Stone.
Both can perform pretty similar functions in battle. Both are low-Speed Pokemon with good bulk and access to a lot of healing moves such as Synthesis, Strength Sap, and the like. Vileplume is a more potent special attacker while Bellossom is better defensively.
13 Whimsicott (Sun)
Another recipient of the Sun Stone for evolution is Unova's Grass/Fairy-type, Whimsicott. While its overall stats besides its Speed aren't great, its abilities and movepool tell a different story.
It's one of the Pokemon blessed with the Prankster ability, which gives priority to status moves. If that isn't your cup of tea, you could run it as an attacker with either Chlorophyll or Infiltrator. Whatever role it plays, it has a deep-enough bag of tricks to play it well.
12 Starmie (Water)
Among other things, Misty's ace Pokemon certainly has some of the best coverage for a Water-type, including not only Psychic-type moves, but nice powerful special moves such as Thunderbolt, Power Gem, Grass Knot, and Signal Beam.
From its three abilities, its best one is probably its hidden one, Analytic, which is a bit of a weird ability to have for a Pokemon with 115 Speed. Otherwise, Natural Cure works great too. All this makes Staryu a worthy recipient of the Water Stone.
11 Honchkrow (Dusk)
Soon to make a reappearance in the Sinnoh remakes after being M.I.A. so far in Sword and Shield, Honchkrow was a sorely-missed offensive presence.
It has just the right set of moves and abilities to complement its high Attack stat. It also has two great abilities for offensive Pokemon: Super Luck and Moxie. To get this Dark/Flying-type bird, give Murkrow a Dusk Stone.
10 Vaporeon (Water)
Jolteon, Flareon, Leafeon, and Glaceon all deserve shout-outs, but of all the Eeveelutions that evolve via stones, Vaporeon has stood the test of time the best. It's proven to have the best stat spread among them, with most of the love going to its HP and Special Attack.
It also has two very good abilities, though one is better than the other. While Hydration's removal of status effects in the rain is pretty good, Water Absorb essentially acts as an immunity to Water-types, taking them with no damage and turning it into HP.
9 Roserade (Shiny)
Roserade has rightfully stood out from the highly-populated market of Grass/Poison-types. It's one of the best special attackers of that mold, boasting a very good 125 Special Attack. It also has a decently wide movepool and a reliable ability in Natural Cure.
It takes a bit of work to get a Roserade, having to evolve Budew via friendship and then give Roselia a Shiny Stone. It's certainly worth the effort though, especially for its potential use in a Sinnoh playthrough.
8 Chandelure (Dusk)
It takes a while to get Chandelure, having to evolve Litwick into a Lampent at level 41 before giving it the Dusk Stone. When it's all said and done, though, you're left with one of the best Fire-types in Pokemon, and certainly one of Unova's best.
It's got an absolutely bonkers 145 Special Attack, an excellent movepool of STAB moves and coverage moves, and a hat trick of great abilities (Flash Fire, Flame Body, and Infiltrator). If you're in need of a strong special attacker, Chandelure's a great option.
7 Togekiss (Shiny)
Fairy-types that work best as attackers are somewhat of a rarity, but Togekiss is all that and then some. Combine its Serene Grace ability with a move like Air Slash, and your opponent will be flinching for days. That's without even mentioning its great 120 Special Attack.
Togepi can be kind of rare, but if ever you find one, evolve it into a Togetic with high friendship. Then give it a Shiny Stone to get the flinch-hax abusing Pokemon of your dreams.
6 Clefable (Moon)
Clefable is arguably the best non-Legendary Fairy-type Pokemon for competitive battling. Its stats are merely an accessory to one of the best abilities in the game, Magic Guard, as well as great moves for attacking (Moonblast), recovery (Soft-Boiled), or set-up (Calm Mind).
There's a strange pattern here; Clefable's another Pokemon that's very hard to evolve into thanks to the high-friendship to stone evolution. However, as seen with the other examples, the time spent evolving this Pokemon will all be worth it.
5 Ninetales (Fire/Ice)
Both regional variants of Ninetales have had pretty productive stints in the higher echelons of competitive Pokemon. The Kanto variant was previously considered the premier sun-setter in competitive Pokemon, and the Alolan variant is considered the premier hail-setter.
Both variants of Vulpix are relatively easy to find, so it's only a matter of leveling it up until it gets the moves you desire, then giving it either the Fire or Ice Stone.
4 Nidoking & Nidoqueen (Moon)
While they are technically two different evolutionary lines, Nidoking and Nidoqueen will forever be intertwined with each other. Regardless, both are great on offense thanks to having access to Sheer Force as an ability, although Nidoking has seen a tad more usage thanks to a slightly better stat spread.
Both Nidoran species are easy to find in the games they appear in, so if you're in quick need of a pinpoint attacker for either a playthrough or competitive team, they're a great bet so long as you have a Moon Stone on hand.
3 Galarian Darmanitan (Ice)
While the Unovan variant of Darmanitan simply evolves by level-up, its Galarian variant requires an Ice Stone. That's no problem, as it's hands-down the best Ice-type attacker out there.
It not only has great stats and moves, but it also has a pair of excellent abilities (Zen Mode, Gorilla Tactics) that are different enough to keep competitive players guessing. Currently, Galarian Darmanitan is banned from competitive play, and for good reason.
2 Magnezone (Thunder)
Magnezone is eligible for this list, thanks to its location-based evolution method being replaced with the Thunder Stone method, and it wouldn't be a stretch to put it at second place.
It has an excellent typing in Electric/Steel. It's got a great stat spread, with a good balance of attack power and defenses. It has three good abilities, with the always-useful Sturdy being king among them. A Magneton is good enough for a playthrough, but if you wanna go the extra mile for competitive battling, Magnezone will not disappoint.
1 Aegislash (Dusk)
There was a time when this Pokemon was so overpowered that it was banned from competitive battling. That's no longer the case, thanks to a slight nerf, but the fact that it needed to be nerfed says something.
What makes this Steel/Ghost-type Pokemon tick is its ability, Stance Change, which switches its stat spread from having high defenses to high attack power, with the help of its signature move King's Shield. To get this powerhouse, find a Honedge, level it up into a Doublade, and give it a Dusk Stone.