Cavalry of any kind. My fav clan to play is Takeda and I always waver when starting a new campaign in STW because half the time I end up picking Takeda and playing the same bloody campaing for the umpteenth time but I still enjoy it.
Of all cavalry my favourite is Yari Cavalry but it toook me literally years to learn how to make a proper use of them. Their charge is devastating and they're the single fastest unit on the battlefield.