Ok, I just crunched the stats. Below are the UUs odds to beat its replacement (ie Praetorian beats a Swordman, etc)
Cossack - 97.10%
Samurai - 94.90%
Panzer - 90.10%
Redcoat - 88.60%
Skirmisher - 80.60%
Praetorian - 78.20%
Phalanx - 75.00%
War Chariot - 75.00%
Immortal - 65.00%
Camel Archer - 62.50%
Navy SEAL - 60.20%
Cho-Ko-Nu - 56.84%
Keshik - 56.80%
Conquistador - 50.00%
Musketeer - 50.00%
Quechua - 50.00%
Jaguar - 27.10%
The list is a bit deceiving. Some units (ala conquistadors, quechua) aren't particularly great against their counterpart, but monsters on other units that usually defend cities. And the Chuck Norris' don't get a chance to show off their collateral damage capability. Also, I didn't take into account that aggressive civs give an automatic +10% to some of their units... the jaguar's don't have that factored in, which would make them slightly more competitive with the unit they are replacing. But I'm afraid that there's no possible analysis that can give the Musketeer any redemption.
And then there's the Fast Workers, which probably aren't going to directly take over any continent... but I'm inclined to consider them the best since they are the only unique unit that you'll use from 4000 BC to 2050 AD. And as someone else said, they can outrun anything.