One of the best parts about the Kingdom Hearts franchise is the massive variety of different Keyblades present in each game. It's a fun objective each time to see all the different designs and try to unlock every Keyblade available. Not all of them are created equally, though, as some have better designs, and some are much some of the best Keyblades in Kingdom Hearts when it comes to gameplay.
The Keyblades in the most recent iteration of the original Square Enix classic Kingdom Hearts are often rated based on two main factors - design and utility. As such, more often than not, the best-looking Keyblades that are also the strongest for use against some of the toughest boss fights in the game will be the ones that shine above the rest.
S Tier
Ultima Weapon: This Keyblade is what everyone who plays Kingdom Hearts is looking for. It easily lands among the very best for having a wonderfully colorful design, looking like it dropped straight out of heaven into the player's hands, and it has the best stats of any Keyblade in the game. It gives a hefty boost to both strength and MP where most Keyblades do just one or the other, and some even limit one while boosting the other. It's a treasure hunt to unlock Ultima Weapon, but that just makes it that much more interesting to obtain.
Divine Rose: This Keyblade falls only just shy of Ultima Weapon in terms of utility. It's incredibly powerful, with high strength and crit stats, but without the MP buff that Ultima gives. It also boasts the beautiful rose design, which is one of the best in the game but is still probably outmatched by Kingdom Hearts' Ultima Weapon for most.
A Tier
Lionheart: This Keyblade sports one of the highest boosts to strength in the game. It's not as high as many of the Keyblades in higher placements, but it also gives a slight MP boost which can make it preferable for some players. Its design is sleek and intimidating, which makes it look ferocious when being swung around. It is one of the harder Keyblades to obtain, though, with the frustrating Kingdom Hearts Cloud and Leon joint boss fight standing between the player and Lionheart.
Lady Luck: This is one of the best Keyblades for players with an "all-around" build, with solid boosts to strength and magic. Being an all-around weapon means it doesn't have any stat that's not outclassed by another weapon, but it evenly distributes good power in a way that will likely appeal to many. Its design is perfect for something that crosses Kingdom Hearts and Wonderland, as well.
Oathkeeper: Similar to Lady Luck, Oathkeepr is a solid all-around Keyblade, with a slightly higher emphasis on its strength whereas Lady Luck has more of an emphasis on its magic. That makes both of these Keyblades equally viable, and which one the player picks purely comes down to whether they want that extra MP bar or a little more oomph to the blows. Oathkeeper's design is also nice, but it does kind of look like a slightly-neutered Ultima Weapon. It's also one of the few Kingdom Hearts Keyblades that get dual-wielded.
B Tier
Diamond Dust: This Keyblade can be great for very specific Kingdom Hearts builds, any player who wants to rely almost solely on magic in their playthrough, but is probably not viable for anything other than that. Getting a +3 to MP is something no other Keyblade in the game can do, but it comes at the cost of having almost no physical strength. It does sport an incredibly unique design, though, that will definitely stand out in the hands of the player.
Olympia: This Keyblade can be a bit of a challenge to obtain, requiring the player to complete the three major cups at Olympus Coliseum, but the rewards are definitely worth the trouble for any player that wants a build focusing on raw power. It boasts high strength and crit, along with a design that looks like it was carved right out of the stones from the Coliseum.
Fairy Harp: This Keyblade is strong, but the most recent version of Kingdom Hearts made it feel a lot more middle-of-the-road in the late game due to a nerf to the weapon's crit rate and bonus. It has a decent strength stat and a +1 boost to MP that keep it in a higher tier, but its fairly bland design isn't doing it any favors.
Pumpkinhead: This Keyblade is one of the most useful for the time in the game when the player can obtain it, with a solid reach, good strength stat, and the ability to land crits easier than many other Keyblades. On top of that, it has a great design that matches perfectly with the reskin Sora receives in the world where Pumpkinhead is obtained. It's not an endgame Keyblade, but it's one the player can get some mileage out of for a while after it's obtained.
Spellbinder: Any player going for a magic-focused build will definitely want Spellbinder, as it's one of the earliest obtainable Keyblades that gives a big boost to magical power and a +2 to the MP bar. It'll likely be replaced later on when better magical Keyblades become available, but it's great for throwing spells around in the mid-game, and its design looks delightfully mystical to boot, even if it looks a little strange to wield, but Kingdom Hearts isn't known for being practical.
Three Wishes: This Keyblade has a sleek and colorful design that will make the player want to use it for longer than it's probably useful for. It's an early-game Keyblade, and definitely worth swinging around for the first world or two after Agrabah, but it gets easily outmatched by most any Keyblade the player can get after the middle point of the game.
C Tier
Kingdom Key: The Kingdom Key is the original Keyblade. The iconic design that began it all, which makes it uniquely special, even if it doesn't have the wow factor or complexity of some other designs. The fact that Square Enix managed to make a large key that players hit people with something to be taken seriously at all is special, and something only Kingdom Hearts could get away with. The main reason it lands among the worst weapons is simply that it has no particular benefits. It's the default key every player begins with, and most will stop using it after getting some Keyblades that have real buffs.
Oblivion: This Keyblade gets design points. It looks ominous, almost alien, and resembles Riku after being fully possessed by Ansem in Kingdom Hearts. Anytime a weapon reduces MP, it takes a bit of a hit in utility. Oblivion does make up for this a little bit by giving a bigger strength buff than other MP-reducing Keyblades, but that doesn't quite save it from being mediocre.
Jungle King: The reach on this Keyblade makes it feel nice in combat, even if it doesn't have much else going for it in the utility department. The design of Jungle King isn't something likely to impress many players, either. It's not particularly bad, but not particularly good either. It's the definition of a middle-of-the-pack Keyblade.
Wishing Star: This Keyblade looks somewhat interesting, if not a little flimsy and lacking in panache. The stats of Wishing Star would make it one of the very worst if it weren't for the fact that its one unique ability makes it the best way to farm Mystery Mold items out of Black Fungus. That makes the Wishing Star strangely essential to almost any playthrough, although it should really never be used outside of farming.
Crabclaw: The stats for the Crabclaw aren't terrible, with respect to the time in the game it can be obtained, and a boost to the MP bar can be invaluable. However, this weapon takes big hits with regard to its design, one of the most impractical Keyblades in Kingdom Hearts, which makes it look more like something that belongs on the wall of a seafood shack than an actual weapon.
D Tier
Metal Chocobo: One would hope that the most Final Fantasy-inspired Keyblade in the original Kingdom Hearts would be a bit more interesting to look at. Sadly, Metal Chocobo is pretty bland, looking slightly like Cloud's Final Fantasy 7 Buster Sword, but in reality more like a piece of shrapnel. The fact that it reduces MP while giving a strength buff that is lower than the strength buff of a very similar Keyblade in the game just makes Metal Chocobo a little obsolete.
One-Winged Angel: The design of One-Winged Angel almost saves it from the bottom, because it does look great, like a flaming sword with complex ornamentation. It's the utility that gives this Keyblade one of the biggest hits of any in the game. It reduces the MP of the player and is promoted as a Keyblade with little power, but devastating crits, which would be great if its critical hit rate wasn't also pretty poor. Combine all that with the fact that the player has to beat Sephiroth in Kingdom Hearts just to get this thing, and it loses all viability that isn't purely superficial.