People typically rank gens by what new pokemon were added. Not the story (though honestly Yellow still stands out with the way it was set up), not the graphics or gameplay (which are obviously constantly improving), and not "would they play it" value.
Competitive folks base it on the stats, typing, and movesets, whereas those like me base it on designs, typing, and RPG possibilities and the like.
Overall I think it's actually fair to say that the best received pokemon were introduced in the first two generations. Back then there was no hate on any particular one because of its design, and for that reason they are regarded as the best. Whether they would be under the same scrutiny as the new generations were they released now is impossible to know. That said, they're sort of grandfathered in as the best introductions to the series and until Nintendo truly, completely, blows them out of the water, they will remain there.
That said, I'm honestly not a fan of Garbodor because, for being trash, it looks more like some demon rabbit. If they did it like trash forming a monster instead of just a trash bag, or actual trash-looking trash in the trash bag, I think it would be significantly better received by many. Not all, but many. And not necessarily liked. But that's just one example of why people put a whole gen over a whole other gen is because at least one pokemon in one simply doesn't live up to the standard set by gens 1 and 2. Even if the rest are enormously better.
"The point of war is not to die for your country. It's to make the enemy die for his."