Water!!!
In addition to being WAY oversaturated (there are more than 10% more water pokemon than Normal pokemon, for crying out loud!!!), their primary weakness (grass) is completely offset by their near universal access to ice type moves, which gives them a combined neutral or better coverage on everything but, of course, water itself! In addition to this overwhelming offensive advantage (with ice, they are the only type of the starter pokemon who naturally inherit an offensive advantage vs dragons, who are supposed to be the big bad scary guys), they also offer the best defensive typing, with only two weaknesses! Nearly a third of all pokemon reside in the field group, but water gets 3 different egg groups! And I know that could be considered a disadvantage, but the fact is it still shows a greater level of thought and care being put into water type pokemon.
The incredible amount of bias given to the water type makes me loathe them, and it gives me Great satisfaction to kill them (which is why I always make it a point to carry around an electric and a grass pokemon). What's outright infuriating is as much as I despise them, I invariably have a water type in my group, because I simply can't afford Not to! In fact, I think you would be hard pressed to find a group Anywhere that does not include at least one water pokemon, other than the very new and very young (making it not only the most common type in game, but also the most commonly played and most common in the meta, as well).
Finally, as I first got Pokemon Red, and have gone with Fire type starters from the very beginning, Water is my natural foe, the bane of my primary pokemon's existence. The fact is, though, with their Ice moves taken into account, the Grass type starters have trouble with them as well, and really, it's not like Grass doesn't already have it rough enough, with more weaknesses than any other type (besides rock, and fewer resistances than rock), so there's that little bit of sympathy loathing going on as well.
I get that Japan is an island, and with water covering 70% of the world, there is a vast amount of sea life to draw inspiration from, making it probably very fun and easy to create all these water type pokemon, and there is variety while surfing to take into account. None of that makes me dislike water any less.