[Longfellow]

@Naix
Growlithe comes at a lower level (relative to where you are in the game) and grows more slowly than Ekans so the difference in stats is less than it seems. They do similar damage the same way that Nidoking and Nidoqueen do similar damage.

I know a casual player won't play like that, but it's a good way to strictly compare Pokémon, which is what we are doing. I'm just trying to be precise and I want to be clear exactly what I'm doing so nobody's surprised when I say that Farfetch'd is extremely powerful or something.

Anyway I've been slow about rewriting Clefairy due to being busy/lazy, so I'll take your kick in my butt and write it now. Also featuring Farfetch'd.

Farfetch'd rewrite

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Farfetch'd (RB) - High Tier
Availability: Spearow can be traded for Farfetch'd in Vermilion City. Spearow themselves are common in the early routes and can be found up to level 17 just outside Vermilion.
Stats: Farfetch'd's base stats are all near 60. Its low stats are somewhat redeemed by the exp. bonus it gets from being an outsider. When you account for its extra levels, Farfetch'd is roughly equivalent to a Pokémon with base 90 attack and its other stats at base 75, which is unremarkable but acceptable.
Movepool: Farfetch'd has a narrow but coherent movepool. It can learn Body Slam, Double-Edge, and Fly by TM and HM—these are its attacks for most of the game. At level 23 it learns Swords Dance, making it much more dangerous against bosses and other multi-Pokémon trainers, and at level 39 it learns Slash, an immensely powerful move that can one-shot many opponents. Agility, which it learns at level 31, is a decent replacement for one of the above moves that allows it to avoid hits and better sweep bosses. Farfetch'd can also make a good HM slave with the unusual combination of Cut and Fly.
Power: Due to its STAB on Body Slam and Fly, coupled with Swords Dance, as well as its late-game Slash (which is unaffected by the Swords Dance attack boosts but does much more damage without setup), Farfetch'd is an offensive juggernaut.
Type: Farfetch'd both hits and gets hit for neutral damage most of the time.
Match-ups:
-Lt. Surge: Raichu OHKO's Farfetch'd.
-Erika: Farfetch'd should win with Fly. It cannot OHKO Victreebel, unfortunately, which leaves it open to Sleep Powder.
-Koga: Farfetch'd can set up on the first Koffing, though Smokescreen can complicate your sweep. Muk's Sludge and Weezing's Selfdestruct will probably kill you if they hit.
-Sabrina: Kadabra and Alakazam are strong enough to two-shot Farfetch'd at least, so if you're going for a sweep you have to hope they choose other moves. Your best strategy is probably to leave Kadabra to one of your other Pokémon and set up your Swords Dance (and Agility, if you have it) on Mr. Mime.
-Blaine: Farfetch'd's newly-learned Slash will ream Blaine's gym, 2HKOing even his bulky Arcanine. You could also set up Swords Dance as usual.
-Giovanni: As silly as it looks, Farfetch'd can Swords Dance on Giovanni's Rhyhorn and sweep his team. It will have to endure residual damage and there are Pokémon who can beat Rhyhorn and Rhydon faster. Farfetch'd can also outpower Giovanni's non-Rock Pokémon with Slash.
-Lorelei: Farfetch'd can set up against Slowbro and sweep through Jynx and Lapras. It can win individually against Dewgong but will get heavily damaged. Cloyster's high defense and Aurora Beam will defeat Farfetch'd.
-Agatha: Farfetch'd must risk various status effects to beat Agatha's Pokémon. It faces additional problems setting up a sweep in that the attack boosts from Swords Dance will increase confusion damage and Golbat can Haze away its boosts. Farfetch'd can usually beat Agatha's Pokémon individually but a sweep is unlikely.
-Lance: Farfetch'd is too frail to survive Lance's heavy attacks.
-Rival: Farfetch'd does not like Alakazam but can win in a pinch if Alakazam does not use Psychic or Psybeam. It also struggles against the sheer power of Gyarados, Blastoise, and Charizard. A Farfetch'd with both Swords Dance and Agility can set up a full sweep on Rival's Pidgeot; if only Swords Dance, then it can set up against Rhydon and hopefully make it to Rival's last Pokémon.
Additional comments: Farfetch'd is a much more serious Pokémon than it looks. It makes up for mediocre speed and bulk and slight TM dependence with its great power. It one- or two-shots most common enemies and matches well against most bosses. Be careful not to exceed the level at which you can control outsiders, though, or your greatest ally can turn into your worst nightmare.

Clefairy rewrite

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Clefairy - High Tier
Availability: Clefairy can be caught in Mt. Moon but are uncommon (highest encounter rate in Red and Blue is 6%). Their levels range from 8 to 12. They evolve via Moon Stone, which is also available in Mt. Moon. Clefairy should be evolved immediately as they do not learn any useful moves by level-up. If the low encounter rate turns you off, you can buy one in the Rocket Game Corner for 500 coins in Red and 750 coins in Blue. It is inexpensive but underleveled, so it is recommended to be patient in Mt. Moon instead.
Stats: Clefable's average stats, high HP, and fast exp. rate make it a Pokémon that is moderately powerful and very bulky.
Movepool: As Clefairy's level-up movepool is barren (though Sing can be useful), it relies on TMs. Fortunately it learns most of them and can mix and match based on the needs of your team. Useful TMs it can learn include Mega Punch, Water Gun, Bubblebeam, Thunder Wave, Thunderbolt and Thunder, Ice Beam and Blizzard, Psychic, Body Slam, Tri Attack, Double-Edge, and Hyper Beam. Some of these moves are both low-demand and extra-tasty, such as Psychic and Bubblebeam. Even if you run a TM-heavy party, however, Clefairy's omnivorousness ensures that it will have a strong moveset with some coverage, though its exact moves vary from run to run.
Power: Clefairy's peak power is in Mt. Moon and immediately thereafter as Clefairy's Mega Punch is your strongest possible move at that point. Due to Normal STAB, Clefairy's Mega Punch, Body Slam, Tri Attack, or Strength will continue to be strong (if not exceptionally so) throughout the whole game. Clefairy can also swing hard from the special side, especially with Psychic which commonly has type advantage in first generation's Poison-heavy atmosphere.
Type: Clefairy usually hits and gets hit for neutral damage.
Match-ups: It is difficult to cover all of Clefairy's many movesets. This write-up assumes Clefairy, as a base, learned Mega Punch in Mt. Moon and continues to use a strong Normal STAB throughout the game.
-Misty: Clefable is the best non-Grass Pokémon at defeating Misty, given its peerless bulk and power at this part of the game. It can take 3 non-critical Bubblebeams from Starmie and can 3HKO it with Mega Punch. This matchup favors you considering Misty likes to mess around with Tackle and Water Gun. Sing and Thunder Wave can also help your odds.
-Lt. Surge: Clefable can withstand 2 to 3 Thunderbolts from Raichu and 2HKO back with Mega Punch. However, Raichu's faster, so you may have to rely on a lucky critical or a bad move choice by Raichu. As before, Thunder Wave or Sing can help.
-Erika: Clefable should be able to power through with its Normal moves, but it risks being trapped by Victreebel's Wrap or Erika's various status moves. If Clefable knows Ice Beam or Psychic, the battle will go somewhat more smoothly.
-Koga: Clefable dislikes the defensive standard of Koga's Pokémon and needs some luck to outpace him in damage. However, Clefable can sweep him. Psychic makes the battle much easier and a status move like Sing and Thunder Wave couldn't hurt, especially against Muk and Weezing.
-Sabrina: Clefable's physical power and special bulk make it an excellent choice for this fight. It can win by the brute force of its Normal moves, though Alakazam can ruin your sweep if it decides to use Psychic.
-Blaine: Clefable should sweep. Rapidash's Fire Spin and Arcanine's Fire Blast hurt, though, and it is possible to lose if Blaine chooses his moves well, which is rare.
-Giovanni: Clefable should use whatever Special move it may have to defeat Rhyhorn and Rhydon as it will not make progress with its Normal moves. Dugtrio, Nidoqueen, and Nidoking can be beaten in a more straightforward fashion, especially if Giovanni spends his time applying Guard Spec. instead of attacking; they do occasionally put up a fight. 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.
-Lorelei: Clefable with Thunderbolt or Thunder can damage Lorelei's Pokémon and, with items such as X Special, even orchestrate a sweep. If Clefable doesn't have Thunderbolt, however, it does not like the physical bulk of Lorelei's Pokémon and struggles to beat more than one in a row.
-Bruno: Watch out for Hi Jump Kick from Hitmonlee and Submission from Machamp.
-Agatha: Even if Clefable knows Psychic, Agatha is a rough battle and you should not expect to sweep her. Without Psychic Clefable can not make satisfactory progress against her Ghosts.
-Lance: Clefable can challenge the Dragonair with its Normal moves but will probably be overpowered if it challenges Lance's other Dragons that way. Thunder or Thunderbolt allows it to defeat Gyarados whereas Ice Beam or Blizzard expediates the rest of the fight.
-Rival: Clefable's chances against each of Rival's Pokémon depends on its moveset. It can always defeat Pidgeot, Arcanine, and Exeggutor. With Ice Beam, Blizzard, or Psychic it can challenge Venusaur and Rhydon; with Thunderbolt or Thunder it can challenge Gyarados and Blastoise; with any of those Electric or Ice moves it can challenge Charizard. With Thunder Wave or Sing it improves its odds against everyone and enables Alakazam to be beaten too.
Additional Comments: Its game-long power through high stats, Normal STAB, and elemental coverage is enough on its own to make Clefable one of the better Pokémon in the game. Its ability to fill any role your team is missing, whether it be a Water Gun/Bubblebeam user to defeat early-game Rocks or Psychic to defeat the Rocket Poisons or Thunderbolt to ream the Flying and Water Pokémon leading up to Cinnabar, is what makes Clefable a truly good teammate.

Edits for clarifying/using words right/etc.