I think a lot of people would very subtly agree with this, and you can tell just how good Gen 3's roster of Pokemon is by how immensely popular many Gen 3 Pokemon are. The Hoenn starters are by far the most beloved set of starters to date aside from maybe the Kanto starters and Greninja. Sceptile, Blaziken, and Swampert are universally extremely popular and soooo many people love those three. Even their base forms are well known as well (and we all know Mudkip has achieved meme popularity). Beyond the starters though, you have a ton of iconic Pokemon in the fanbase. There's Gardevoir, Aggron, Flygon, Absol, Milotic, and of course Salamence and Metagross as standouts who have massively popular fanbases. Kyogre, Groudon, and Rayquaza are also arguably among the most well received cover legendaries of the lot. Let's not forget how well known the Regis and Latis are, the Regis so much so that we got Regigigas a gen later and even as of late new members to the Regi group.
While the likes of Gens 1 and 2 had a whole lot of dud/not good Pokemon, Gen 3 by far has the biggest fanbase in terms of its Pokemon. Pretty much everyone at least knows many of those Pokemon and there are many of those that have become memorable and iconic over the years and are massive fan faves. It's true that as much as I love say, Gens 4 and 5, even they don't have as many Pokemon that have reached the level of iconic that many Gen 3 Pokemon have achieved.