Clefairy - Low Tier
: Clefairy can be caught in Mt. Moon, but are quite uncommon there (highest encounter rate in Red and Blue is 6%). Their levels range around 10, with 8 being the lowest and 12 being the highest. They evolve through the use of a Moon Stone, 2 of which happen to be in Mt. Moon, the place where you find Clefairy in the first place! The first one is in the northwest corner of the first floor, and the second is in the bottom floor, hidden 4 steps below and 6 steps to the right of the Super Nerd.
Of course, if you don't want to go on a Clefairy hunt but still want one on your team, you could buy one in the Rocket game corner for a low cost that you should be able to afford (500 coins in Red, 750 coins in Blue, not there in Yellow), but it'll be underleveled in comparison with the rest of your team when you get it meaning you're better off looking for them in Mt. Moon earlier on in the game.
You can evolve Clefairy whenever you want, but it's recommended that you do so once you get a Moon Stone, as Clefairy's level-up movepool leaves quite a bit to be desired.
: Clefable's stats are average in many areas, with the exception being HP, which is high enough for Clefable to take repeated hits. Unfortunately, its lowest stat (aside from its "good enough" Speed) is its Attack, meaning that despite getting STAB off of Normal-type attacks, Clefable won't have much business using them as opposed to a move from its vast special movepool, such as Thunderbolt or Fire Blast.
: Clefairy's level-up movepool is, simply put, bad. Moonlight, the only reason why people would delay evolving Clefairy, doesn't exist yet, thus leaving absolutely no reason for not evolving it immediately outside of "I'm also using a Nidoran and don't know where the hidden Moon Stone is". The problem with this, though, is that because Clefable learns no moves by level-up, making it completely dependant on TMs to keep up. Among these TMs are Water Gun, Mega Punch, Thunderbolt, Psychic, and more, but the problem with all of this is that the TMs that benefit Clefable the most are one-time use TMs (Yes, Mega Punch can be purchased in Celadon City, but by then your Pokemon should have equally powerful/more powerful alternatives for it.), and as such Clefable will face competition for a large amount of them. However, Water Gun is almost necessary early on if you want Clefable to be useful for Mt. Moon and Rock Tunnel. Mega Punch is also quite helpful as a general attacking moves.
: Clefable is quite good in the first half of the game if you give it the right support (read: Water Gun, Mega Punch, Moon Stone), but after Rock Tunnel, Clefable quickly fades into obscurity as it stays the same (unless you use more one-time TMs on it) while everybody around it gets better.
: Normal could be considered the "blank slate" of all the typings, being weak to only Fighting-type attacks, which only comes into play for 2 battles in the entire game, and one of them is optional, while it has no resistances or immunities save for Lick (The other Ghost-type moves, Night Shade and Confuse Ray, aren't affected by type match-ups), which is only appears in Pokemon Tower.
: The following match-ups are assuming you taught Clefable at least Mega Punch, as it does dismally against all of them otherwise.
: Clefable has the bulk to take 3 Bubblebeams from Starmie (assuming none of them crit, of course) and its Mega Punch is powerful enough to beat Starmie in 3 hits, meaning that if Starmie messes around by using Tackle or the like, Clefable wins.
: As with Misty, Clefable can withstand 2 Thunderbolts from Raichu and 2HKO back with Mega Punch. The problem is, Raichu's obviously faster, so Clefable either has to get lucky with a crit or Raichu has to use Growl or Thunder Wave.
: Cefable's slower than her Victreebel, so it's inevitably going to get trapped by Wrap if it chooses to use it. Tangela and Vileplume are more manageable, especially if Clefable knows Ice Beam. Watch out for Sleep Powder from Vileplume, though.
: Simply put, if Clefable knows Psychic, it wins, but not smoothly. If it doesn't know Psychic, Koga wins.
: Clefable can beat all of her Pokemon by itself with the exception of Alakazam. It can do decently against it if it didn't fight anything else in her party, but doesn't have a guaranteed victory over it.
: Fire Blast can hurt a bit, but Clefable should still be able to beat Growlithe and Ponyta by itself.
: Clefable needs Bubblebeam to stand a chance against him, as Water Gun just does not have enough power behind it to OHKO even his Rhyhorn. Of course, you shouldn't expect Bubblebeam to OHKO either of his Nidos, but at least it's better than only getting a 3HKO against them. In Yellow, his Nidoqueen and Rhydon can actually pose an offensive threat, making things overall more dangerous for Clefable, who will get worn down quite quickly when fighting Dugtrio and Persian alone.
: For Clefable to beat her, you need 3 things:
2. An X Special
3. At least 2 Hyper Potions.
: Watch out for Hi Jump Kick from Hitmonlee and Submission from Machamp. Bruno's Onix are pathetic and Hitmonchan can be easily dealt with if you don't use Mega Punch, Strength, or any other Normal-type attack, as it can Counter it back at you.
: Her Ghosts take pathetic damage from all of Clefable's possible attacks (except for Psychic, of course) while Arbok (in Yellow) knows Wrap, which can trap Clefable for an extended period of time, meaning the only Pokemon Clefable should even consider taking on is Golbat.
: While Clefable's bulk allows it to take a non-critical Hyper Beam from any of his Pokemon, it can't really do much back to them unless you taught it Thunderbolt (for Gyarados) or Ice Beam/Blizzard (for everything else).
: Whatever Clefable can beat can be easily accomplished by almost every fully evolved Pokemon in the game, so its performance against him isn't too impressive (for those of you wondering, it can beat Pidgeot, Rhydon, and Exeggutor).
: Simply put, Clefable is one of those Pokemon that's good early on in the game, but falters mid-game and never gets out of its slump. It also needs a large amount of support to be good, and its rarity certainly doesn't help matters much.