How on earth has no-one mentioned Paras(ect) yet?
If there was a poll done for weakest evolved Pokemon ever, Parasect might well qualify. A lacklustre stat spread coupled with quite possibly the worst typing in the game was never likely to add up to good results. Bug/Grass is a spectacularly poor typing which results in not one but two 4x weakness - Fire and Flying, two very common attacking types, and a typing which manifested itself twice in Gen I in the form of two very powerful and popular Pokemon. The odds were against it from the start.
Bug/Grass didn't end up being unique to the Paras line. The Sewaddle line and Wormadam-G share the typing, but that's it (which is a surprisingly small number, really. Bug and Grass are pretty similar typings, so you'd expect to see them intersect more often). But, by dint of having been around since Gen I, Paras and Parasect are the only Pokemon to ever have 3 double weaknesses. Yes, in Gen I, Poison was super-effective against both Bug and Grass, meaning that the Paras line was doubly weak to Fire, Flying, and Poison. Nasty stuff.
While Parasect's moveset in Gen I wasn't as terrible as the vast majority of Pokemon in that era, it's still far from great. It didn't learn a single move with over 80 base power on its own; Gen I's janky TM list has just enough options to cobble together a passable moveset, but the only decent STAB move it got was Solarbeam, which had a pretty huge drawback. There weren't any decent Bug moves back then, so that was it.
Later gens added a few new fun tricks to Parasect's arsenal, but none of them were good enough to pluck it from obscurity. They remain one of the few remaining Gen I families to never get any serious alterations or upgrades; while Gen VI graciously buffed the base stats of a few older families, Parasect got nothing, and it's never had a Mega Evolution, regional form, or further evolution. It has always felt, to me at least, like a distinctly unglamorous Pokemon family. Never hugely popular, and never used by any prominent NPCs. Even Pokemon Go has snubbed it - it was, along with Spearow, the Hitmons, Ditto, Snorlax, and Mew, the final Kanto family to be made available shiny, and notably has been virtually nonexistent in the wild for long stretches of the past two or three years outside of events. Which is incredibly frustrating because I think shiny Parasect looks fantastic and I'd like to hunt it.
And oh, of course it was Dexited. Frankly I expected nothing less.
But for all this, I do think Parasect has a lot of redeeming features. It does have a somewhat usable Attack stat, but it's pretty clear that Parasect was meant to be the bridesmaid, not the bride - it's the ultimate team supporter, especially in Doubles. It learns an absolute grab-bag of interesting moves with tricky side effects, including a truly stunning array of Egg moves like Grassy Terrain, Rototiller, and Wide Guard, and is perhaps most famous for being one of the small handful of species with access to Spore. The biggest boon it ever received, IMO, was getting Dry Skin for a second ability in Gen IV. Sure, it isn't gamebreaking, but it's a pretty neat ability, and definitely its best choice - well, Fire attacks are already OHKOing it, so increasing the damage they do isn't much of a heavy price in return for an immunity and passive healing under the right weather. It's a nifty ability which actually makes it a semi-usable and funky choice on Rain teams in conjunction with its supportive options.
It's also just a genuinely cool design which is both cute (in a certain light) and ever so slightly creepy - and the lore behind it is interesting, mimicking real-world parasitism as it does. Hell, it's actually pretty fun to use in-game. I used a Paras on my last run through HeartGold and had a blast with it... but then I do enjoy using weaker species for a bit of extra challenge, so you can judge that for yourself.
But yeah. For such an objectively poor Pokemon there's a lot to like about the Paras line. If nothing else, I just love rooting for the underdog. Finding the beauty no-one else can see... you get the gist. It's legit one of my favourite Gen I mons.