AND they're not even good competitively so didn't even have the chance to leave a mark.
This is a huge point that I think a lot of people gloss over. I've never really given much of a shit about something like say, Venusaur, but after so many years of being around it and having it win me so many competitive games it's odd how it slowly grows on you. It's kind of endearing to me now even though I don't care for the design much. I wonder if Smogon had a favorite Pokemon contest if it would differ in results from the more general favorite Pokemon quizzes on Reddit or wherever.
Speaking of Venusaur, I think it's the weakest Red / Blue starter. Worth mentioning is that R/B is easy and all the starters can easily solo the game but Venusaur is slightly the worst at it despite being considered "easy mode".
Venu only learns Vine Whip and Tackle until level 30 where it gets Razor Leaf. While Razor Lead is good this is effectively the only good move it learns period. The only TM it gets that actually matters is Body Slam which is good, but Venu has the worst attack of the three starters so it's weakest. There's also Mega Drain which is okay, but very weak and has terrible type coverage against most opponents. I guess that just means it can learn its only HM Cut with less of a moveslot penalty? Compare it to the other two...
Charizard gets Slash and Flamethrower via level up and Earthquake / Dig / Fire Blast / Fissure (X-Accuracy makes it 100%) via TM. It gets Cut / Strength via HM.
Blastoise has a pretty bad level up movepool but it does eventually get Hydro Pump. TMs make it work well as it gets Bubblebeam from Misty as well as Ice Beam, Blizzard, Submission, Earthquake, Dig, and Fissure. For HMs it gets Surf and Strength.
It's terrible against the majority of the bug / bird Pokemon in the early routes.
It's probably the worst against Brock. Squirtle solos him at level 8 with Bubble. Charmander does it around level 10-11 with Ember. Bulbasaur has to be 13 to get Vine Whip, unless you want to try it with Leech Seed + Tackle.
Cerulean City is arguably the hardest part in the game as you're hard walled between Misty and the Rival battle. Ivysaur does well against Misty but it's pretty awful against the Rival, only really being useful against Ratatta / Abra (everything is good against Abra). Charmander / Squirtle can usually solo the rival's first three, use the rest of the roster against his starter, then grind nugget bridge until you can push through Misty. Crushing Misty is easily Bulbasaur's highest point. Also worth mentioning is after the rival battle you can capture Oddish / Bellsprout both of whom learn Razor Leaf and have similar stats to Venusaur.
Vermilion City's Surge is soloed by catching a wild Diglett, or even by just using Dig a few times on your starter. Well except Ivysaur who has to grind through it with Vine Whip! The rival is on the SS Anne and once again matches up better against Ivysaur with Kadabra and Pidgeotto.
Assuming you don't skip it for later, the Lavender Tower Rival is pretty rough for Ivysaur. 4 of his 5 Pokemon resist grass and two hit you super effective. Wartortle and Charmeleon only have two bad type matchups.
Celadon city gives you Fly, allowing the Doduo you caught to solo Erika. Alternatively catch an Oddish / Bellsprout and teach it Cut as her shitty AI will spam Poisonpowder until you team wipe her. I guess Cut Ivysaur solos without issue. Venusaur gets Mega Drain here so that's something but also Blastoise gets Ice Beam. While Venusaur matches up typewise against Giovanni well he also has an Onix + Rhyhorn that ONLY KNOWS HORN ATTACK. It doesn't even matter what you use. His only "real" Pokemon is Kangaskhan who all starters take on the same.
The Ghost tower sucks balls for Venusaur. You might have Razor Leaf OHKO the Gastly here if you're high enough level but Char and Blast definitely one shot everything here.
At this point I usually go make a deal with the Missingno and become a god but assuming you're following the path next up is Saffron City. As per usual Venusaur matches up bad against the rival (he can't even hit Gary's water type Super Effective!), bad against the average poison type Rocket grunt, and is just total, incomparable trash against Sabrina. I guess Venusaur does decent against the fighting gym, but the Hitmons have 35 special lol so it's really not much of a fight for FWG STABs.
What's next? Cycling road or the route south of Lavender to go to Fuchsia? Cycling road has a lot of poison type bikers while the Lavender road has lots of bird trainers and cooltrainers which grass sometimes stumbles against but fire / water sweep.
Venusaur gobbles ass against Koga and his psychic-juggler buddies. Char / Blast both probably know Dig and easily handle Koga.
I think everyone usually flies to Pallet to take the easier route to Cinnabar but once you're there get ready for a fun time of poison / fire types in the mansion and getting just destroyed by Blaine. Char / Blast easily sweep. Also Missingno Sky Attack smashes Venusaur.
Lastly there's Giovanni. You'd think the grass type would be the best against the Ground type gym leader but... not really. Blastoise obviously 6-0s but Charizard is immune to ground and none of them have rock moves. Most of them have special stats hovering around 45~ so Charizard likely one shots the majority of his team. Venusaur also probably sweeps with Razor Leaf. It's just an easy fight in general.
On the way to Victory Road you fight your Rival. Squirtle matches up well against 5/6 of his Pokemon, Charizard matches against 5/6 Pokemon (Rhyhorn only knows Horn Attack), but Venusaur is only good against 3/6.
Victory Road is full of a variety of things but if you go after Moltres, oops! Venusaur is weak. Oh did you try to catch Zapdos? Drill Peck. Ooh, Articuno? Ice Beam.
Elite 4 Lorelei is on paper hard for Venusaur but probably easy if you can OHKO everything with Razor Leaf. Blastoise also does well but Charizard is worthless unless you use Fissure + X-accuracy.
Bruno loses to anything with a pulse.
Agatha only has poison types. 4 of her 5 Pokemon hit Venusaur super effective.
Lance gets soloed by anything with Ice Beam but if it comes down to it Venusaur doesn't really do much to dragons. I think Lance will spam Agility due to psychic type "super effective" so I guess Venusaur can spam Cut until you win. At least Charizard has Slash.
Rival gets pretty much soloed by Dig Charizard / Blastoise but Venusaur loses badly to Pidgeot, Alakazam, and Charizard.
Beat the game and go to the Cerulean cave to get your grass / poison ass blasted by Mewtwo. Or Master Ball it.
tl;dr Venusaur sucks against almost everything in R/B