Sejong is my choice mainly. First tier only 'costs' 2 citizens and I got all of them to at least lvl 5 so that I can continue with 2nd tier. My first bigger aim here is protect tactic, which is his 3rd tier research with (I guess) 4 citizens. A 30% to protect tactic time would be a great boost and I estimate that it would last for 1min if fully upgraded. I could then use my second protect tactic for something else, maybe.
Of course, the ultimate goal would be to fully upgrade Sejong for the 1 citizen research reduction. Being at 22/100 though, I estimate, it would take years to finish.