I originally said ghost, but I have to change my answer to ice. Seriously. Ice is SO forgotten to me as a type, that I forgot about it when I made my first post, but remembered it when I was looking at a type chart today. Most of the ice types in the game are mixed with water, so it's easy for me to forget they are a type. Like, I use ice beam all the time, but I forget that there are actual ice POKEMON as well rofl. What is there that's not also water? Snowrunt and it's evos, Smoochum and Jynx, and um Snover and Abomasnow? Are there any more non-water ice types? I seriously can't process ice being a pokemon type LOL, I'm so to use ice attacks on non-ice types.
I remember when Red & Blue came out, the strategy guides and stuff said "USE FIRE POKEMON AGAINST LORELEI BECAUSE SHE'S ICE" I was like WTF? Even in my first playthrough I wasn't that stupid O_O You could use a fire type against Jynx, but that's it.