Merciorum
Prince
1) Kris Swordsman
2) Winged Hussar
3) Immortal
4) Siege Tower
5) Janissary
2) Winged Hussar
3) Immortal
4) Siege Tower
5) Janissary
I didn't list the Impi only because it's not so much the Impi, but the Ikanda (the UB) that makes Zulu melee units amazing.
1) Kris Swordsman
2) Winged Hussar
3) Immortal
4) Siege Tower
5) Janissary
This isn't really fair because part of what makes the Keshik so insane is Genghis Khan's UA and the Khans themselves who can keep up with them. That's not to imply that slightly slower Keshiks without a perma GG who also provides +15 healing would be bad but that's still a non-trivial part of what makes the Keshik storm unbeatable.
Similarly the Impi itself is nothing to scoff at (double XP, free first strike attack, upgrade to rifleman) ... but obviously means a lot more in light of all the buffalo parts: +1 move, +10% strength, +10% open terrain strength, +75% flank (really just +7.5%), +30% ranged defense. And of course there's the 50% maintenance cost.
Comparing UU's without the context of the Civilization they come from isn't of much use when those civ's grant unique combat bonuses of some kind. It's not like you can play Ghengis without Khans and Mongol Terror and if you are building Impi's without an Ikhanda you're doing it wrong.
Impi are bar-none the best non-ranged unique unit. The Ikanda and Honor (Autocracy too if you make it to that) are just icing on the cake.
5. World Congress