For me it's gotta be Naginata Samurai.

I almost never used them in the original shogun, preferring the No-Dachi/Yari samurai charge to anything else. But I find myself really enjoying the Nagitana in shogun 2, i find it a very versatile weapon in the current game balance. While not as effective against spears as swords, and not as good against cavalry as a yari, i still find it a great all purpose weapon. If my line looks like it'll breach then its time to send in the Nagitana to reinforce and get the job done.

What are some of the units you find are making it into your armies, perhaps more than you thought?