Now, I don't have Sugoroku, but I'd have to say the Masterwork War Yumi dominates. Sure, you have to spend a huge amount of precious attribute and skill points just to be able to use it, but once you get there, and you have high archery proficiency and ceremonial arrows, you will always kill an enemy with one or two hits, at any range, through any armor. If you have two large bags of ceremonial arrows, that's sixty-four shots. You won't be able to have as great an army as you would if you invested points in other skills, but you will become the god of war.
If you're unwilling to sacrifice that many points, then some kind of kanabo/tetsubo is excellent. Armor-smashing, block-crushing, freakishly powerful, and able to net you some captives. I favor the Balanced Practice Kanabo because it's the fastest club, has no strength requirement, and can still crush through blocks and do a lot of damage.
The katakama yari in older versions of Gekokujo was excellent since it was a quick, hard-hitting spear with good reach and a fearsome, armor-piercing swing. Now it can't swing.
Naginatas are also pretty fearsome. Long reach and a frightening cutting ability that somehow gets through heavy armor. I always target naginata-armed enemies first, since I've noticed they have a tendency to carve up my armies when they close.