One thing that factors in to the 'best unit' is whether you're fighting human or AI; praetorians are much better against the AI than humans because the AI won't build tons of axes to counter them the way a human will.
I think Cossacks are stronger overall than redcoats because they come so much earlier on the tech tree. It's pretty easy (at least on monarch) to get liberalism first for free nationalism, then research military tradition and research/trade gunpowder to have cossacks when your opponents only have pikemen as a counter. To get redcoats, you have to go further up the tech tree and in a less natural direction.
Cho-Ko-Nus are OK, they're good at fighting in the field but are very poor at actually conquering territory unless you pull off a rush, and crossbows are basically as good at that rush. They certainly don't shred everything in their era; they're equal against one-era-back horsemen, weak against one-era-back elephants, trashed in the field by same-era knights, equal against cheaper same-era longbows in the field, beaten by longbows in cities, and only slightly ahead of macemen (roughly equal against aggressive maces). They're not a useless UU, but I'd much rather have samurai.