S-Tier: N/A
A-Tier: Heracross
B-Tier: Pinsir, Scyther
C-Tier: Caterpie, Scizor
D-Tier: Pineco, Venonat
E-Tier: Ledyba, Paras, Shuckle, Spinarak, Weedle, Yanma
I'll quote the full analyses for the top 3:
Heracross:
Availability: Early-game (Azalea Town (Headbutt), levels 2-5, 30%).
Typing: Bug / Fighting is handy against Whitney, Jasmine, Pryce, and Karen's Dark-types, but is outright bad against several opponents like Team Rocket, Will, and Koga.
Stats: Heracross packs a great Attack, coupled with mostly average other stats.
Movepool: Heracross relies on Headbutt through tutors and Aerial Ace until it learns Brick Break at level 19, which becomes its main STAB move. Close Combat at level 37 provides an even stronger STAB move and Shadow Claw and Earthquake through TMs provide coverage. Counter at level 25 can be used in niche cases against strong physical attackers (like Bruno's Machamp).
Major Battles: Heracross handles Whitney effortlessly with Brick Break and performs well against Morty with Shadow Claw. It can easily KO Karen's Umbreon and generally hits hard any opponent it can hit neutrally.
Additional Comments: Heracross is a strong Pokemon that is handy against some of the most difficult opponents. However, its Slow growth rate and low join level, coupled with a few bad matchups, prevent it from being perfect. In Azalea Town, there are two types of trees, of which only one has Heracross; Heracross can be found on ones where Spearow and Aipom come at levels 3-5.
Pinsir:
Availability: Early-game (National Park (Bug-Catching Contest), level 13 or 14, 5%).
Typing: Bug is helpful against Will and Karen, but before that, it doesn't see much use offensively as Pinsir learns X-Scissor somewhat late. It is useful for resisting Chuck and Bruno, though.
Stats: Pinsir is a physical tank with an excellent Attack. Its other stats aren't particularly impressive, though 85 Speed lets it outspeed most of the game.
Movepool: Pinsir relies on Dig through TMs and Seismic Toss until it gets better moves, such as Brick Break at level 21 and X-Scissor at level 30. Earthquake through TMs is an upgrade from Dig and Swords Dance at level 38 helps Pinsir sweep end-game fights.
Major Battles: Pinsir can beat Whitney's Miltank with Revenge as long as it doesn't get flinched. Morty can be swept with Dig if Pinsir has Mold Breaker. It can OHKO Jasmine's Magnemite and beat Pryce's Seel and Dewgong. At the Elite Four, Pinsir sweeps Bruno easily by using Hitmontop as a set up fodder and crushes Karen, notably OHKOing Umbreon with +2 X-Scissor.
Additional Comments: While Pinsir is decent for some key fights, it is very difficult to obtain due to its low catch rate and rarity, which is made worse by the fact that you want one with Mold Breaker to perform well against Morty. Furthermore, it is in the Slow experience group, so it's a chore to train too.
Scyther (not Scizor):
Availability: Early-game (National Park (Bug-Catching Contest), level 13 or 14, 5%).
Typing: Bug / Flying is very useful against Chuck, Will, and Bruno, but is very bad against Jasmine and Pryce.
Stats: Scyther is a fast and strong physical attackers which can easily outspeed and KO most opponents before they can touch it.
Movepool: Scyther will mainly use Wing Attack (learned at level 21), which hits very hard thanks to Technician. X-Scissor at level 41 helps out against Will and Karen's Umbreon. Rock Smash through HMs provides Scyther with some coverage against Rock- and Steel-types.
Major Battles: Scyther performs well against almost any opponent it can hit neutrally with Wing Attack, including Whitney. Scyther also has an acceptable matchup against Will and Karen with the help of X-Scissor.
Additional Comments: Scyther is incredibly difficult to obtain, as it is hard to catch, has a low encounter rate, and requires Technician in order to perform well. This offsets greatly its all-around high strength.
Hope I helped!
Source: Fizz's more recent link to Smogon's HGSS In-Game Viability Writeups, but the one I provided in the comments has the same text for these three, thx MB.
