I love Samurai.
Their ability to fight until their last point of health with full force makes them absolutely devastating. Especially in MP, when some opponents seem to forget that fact. You can fight much longer before retreating and you often lure your opponent in fights they thing they can win. You'll get them in medieval-age, when everyone else has only ancient walls. Buy them with faith (Japan has its holy sites 50% faster), accompany them with a ram / siege tower and off you go. You won't need catapults.
I also love Minas Geras.
They wreck not only your enemies fleet, they also cut through city-defences like a knife through warm butter in that crucial phase, when you don't have bombers yet, but the enemy already has steel- or renaissance walls. Since they come with the nationalism civic, you'll be fighting with this battleship with 3 range, when everyone is still reliant on frigattes with 2 range (unpromoted, for sure).
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In general you make most of UU, if you get them early, so the window of time extends during which they are superior to equivalent non-unique units of the same age. So beeline.