Find your niche.
First off, very single starter of the Water-type is awesome. The HMs typed with good 'ol H2O are useful and immensely powerful, if not the strongest of the bunch. Even without that, Water-type legendaries like Kyogre never fail to be awesome. Water-type Gym Leaders are all cool, too. Most importantly, my favourite Pokémon of all-time is a Water-type. I don't know what other type I would pick over it as my favourite, so this question is easy to answer.
It's Fire. Wait no, I've always been a Water-type fan so Water it is.