Ultimate
noun, の adjective
You are trying to research the extremes. If you do that, you'll be able to figure out the ultimate weapon. Just researching the safe middle-ground stuff won't get you ultimate. Extreme research, however, will.
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.
あの榊氏までもが、私が宇宙の究極の原理を解明したことに祝いの言葉を述べてくれました。
Even Mr. Sakaki congratulated me on finding out the ultimate fundamental truth of the universe.