My personal favorites is 1, followed by 3.
I feel like the first game feels so laid back and chill compared to 2 and 3. The new entries feel more geared toward splatoons competitive scene/part of the fanbase. but between 2 and 3 I would say that 3 is more balanced gameplay-wise.
speaking on story modes its pretty easy to say octo expansion takes thee cake, and 3's story mode is definitely something new compared to 1 and 2's although the side order DLC left me kinda disappointed.