-Caterpie - Low Tier
-Availability: Caterpie are quite easy to obtain. They're indigenous to Viridian Forest and are quite common there in all games except for Red. Caterpie can be caught at level 3 in Red, level 3 through 5 in Blue, and level 3 through 6 in Yellow. Their first evolution, Metapod, can also be caught in Viridian Forest, but their only move is Harden, making them completely useless until they evolve to Butterfree and learn Confusion. Speaking of evolving, Caterpie evolve at level 7 and then level 10.
-Stats: Butterfree's stats, while great for the first portion of the game, are unimpressive in the long term, with its best stats being Special and Speed, neither of which are particularly impressive. Being your garden variety caterpillar and crysalis, Caterpie and Metapod are far from threatening at any point in the game.
-Movepool: Something important to remember is that Butterfree learns all of its level up moves ,save for Psybeam, earlier in Yellow than in Red or Blue. For example, Butterfree learns Confusion immediately after evolving in Yellow, while it has to wait until level 12 in Red and Blue. It gets access to Sleep Powder and Stun Spore at a level similar to Bellsprout and Oddish, in other words before level 20. After Confusion and the powders, Butterfree's level up movepool stagnates, learning nothing of note until level 32 (level 34 in Yellow), when it learns Psybeam. A TM move you will definitely want to teach Butterfree is Psychic. It's Butterfree's most powerful attack in most circumstances, and as such it is highly recommended that you teach it to Butterfree. Another move Butterfree would appreciate learning would be Mega Drain, as while it isn't as powerful as Psychic, it helps Butterfree against the late-game Water-types and assists it when fighting certain later bosses.
-Power: Caterpie's not exactly the most dangerous tool in the shed until it evolves and learns Confusion. After that, though, it does quite well until S.S. Anne, which is where its power starts to decline, and it becomes reliant on Psychic to deal damage to its opponents, as Confusion won't be OHKOing that isn't a Fighting-type. Psybeam helps, but it doesn't grab any notable KOes Confusion doesn't outside of Koffing, Zubat, and other base form Poison-types.
-Type: Bug/Flying is easily one of the worst typings for a Pokemon to have in Generation I from a defensive standpoint, as it's weak to the STABs of the common Poison-types and Flying-types. It isn't much better offensively for Butterfree in particular, as it's primarily a special attacker and as such can't use its STAB to good use, even if it knew any.
-Match-ups:
--Brock: In Yellow, Butterfree is one of only four Pokemon who can beat Brock reliably and without worry. Due to the fact that it has to wait until level 12 to learn Confusion in Red and Blue means it isn't as likely to do well against him.
--Misty: Butterfree can take a Bubblebeam from her Starmie and paralyze it or put it to sleep, but it can't do much else.
--Lt. Surge: Due to its typing, Lt. Surge's Raichu hits Butterfree super effectively, frazzling it.
--Erika: By this time, Butterfree should know Psychic. With it, Butterfree can defeat Erika handily, thanks to her entire team (save for Tangela who isn't much of a threat) being hit super effectively by it.
--Koga: Despite being weak to Psychic, his Pokemon can withstand a blast and respond with Sludge. In Yellow, his Koffing, Muk, and Weezing are replaced with 3 Venonat and a Venomoth. Because Butterfree is weak to Poison-type attacks, Toxic should be the only move Koga's Pokemon would use on it, but keep in mind Psychic's mere 10 PP.
--Sabrina: Much like with Misty, Butterfree can status her Pokemon, but not much else.
--Blaine: By virtue of typing (and stats), Butterfree loses here.
--Giovanni: Butterfree beats Giovanni in Red and Blue, but only if it knows Mega Drain. Psychic definitely helps it, but it isn't necessary in this scenario. In Yellow, Persian gives Butterfree trouble, and Nidoking and Nidoqueen both have a new technique under their belts: Thunder.
--Lorelei: Lorelei's Pokemon hit Butterfree super effectively, making this match-up clear.
--Bruno: This match-up is the opposite of Lorelei's, provided Butterfree knows Psychic and Mega Drain. The former allows it to beat his Fighting-types and the latter lets it crush his Onix.
--Agatha: Even with Psychic, Butterfree has trouble here due to her Pokemon being faster and having superior Special.
--Lance: All of his Pokemon have Hyper Beam, and considering Butterfree's paper-thin defenses...
--Rival: In Red and Blue, Butterfree can take on Rhydon, Exeggutor, and Venusaur, while Exeggutor is the only Pokemon on his team in Yellow that is seriously threatened by Butterfree.
-Additional Comments: Without a doubt, what holds Butterfree back from being a force to be reckoned with long-term is a combination of lack of STAB and the posession of a merely average Special stat. Because of those 2 factors, Butterfree has a disappointing power output in the game's later stages. That said, Butterfree is a very capable early-game Pokemon. Its access to Sleep Powder and Psychic-type moves, which have superb type coverage, make it a valuable asset before your team gets filled out, the former making it great for catching the rest of your team, notably Pokemon like Abra. Butterfree is notably better in Yellow, due to its positive match-up against Brock as well as its earlier access to Confusion, but that doesn't change the fact that it still has power issues as the game progresses. Another thing to keep in mind is that outside of status, Butterfree is largely dependant on TMs to defeat the later bosses, and is TM hungry in general after Rock Tunnel.