I agree with elmurcis on chain-irrigation making Civil Service a top-tier Medieval tech. That can really transform some maps and is almost always at least somewhat useful.
Feudalism is next for me. Yeah, I know, everybody whips, but I like my serfdom for faster workers, and combined with newly being able to chain-irrigate, it can speed up the transformation of the empire into an agricultural wonderland. Vassalage can be nice for the XP, too, it can mean an extra promotion which is quite useful. And while I'd rather not be on the defensive, when I am, Longbows are a game-changer.
Conversely, Feudalism also wins points because when the AI gains it, they become much tougher to conquer. It's about at that point that one could really use...
Engineering, for Trebuchets to take cities more easily. Faster roads are nice, too.
Didn't vote Philosophy as many have, as for me it's more the means to an end (Liberalism) rather than an end in itself. I'm rarely fishing for a religion by the time Taoism arrives, rarely have enough Priests to bother building Angkor Wat, and Pacifism is situational. Certainly there are a fair amount of times I'll research it relatively early, but not for the sake of researching it.
I agree that many of the others are situational in priority, and Divine Right is usually the least important.