America's UU should be the stealth bomber with a 75% evasion chance.
Everyone else should get something like a B-52/TU-95 bear, or a TU-160 with no evasion chance.
Nothing in the world compares to our B-2, all other bombers are like our B-52.
Not much compares to the B1-B- sure the TU-160 was a mach 2 heavy bomber like the B1-A, but the B1-B was redesigned for low level penetration of airspace, canards added, terrain following radar, more stealth features, etc.
Heck, since america gets it UU so late, it should get more of them so they can actually make a difference.
A UU carrier- holds 4 fighters, 24 strength(a transport should not = a carrier in strength)
A UU bomber- 75% evasion chance, +1 range +4 strength, remove evasion chance of other civs and change theirs to a Tu-95 model
A UU fighter - F-22 +1 range(cause of economical supercruise), 30 strength, 50% evasion chance, 100% intercept chance. This would not replace the Jet fighter, but would be in addition to it, and more expensive.
Giving the US definite air superiority.
The US being the only one with a stealth bomber could be a game changer late game, particularly with a higher evasion chance1
Or give them an earlier UU- the P-51, sure wasn't "That" unique, but its more unique than our navy seals (The SAS would probably be pissed should they learn of the Navy SEALS glorification is civ) - the P-51 was the longest ranged fighter of WW II- give it +3 operational range compared to a normal fighter, and start with a promotion since P-51 pilots were better trained then other countries pilots who had to rush pilots through flightschool to meet demands.
Heck- make them the tuskegee airmen - they were the best trained pilots of WWII they had really hard training because they either wanted the blacks to drop out, or didn't want the first blacks to do bad - either way, they were legendary in battle - not a single bomber was lost to enemy fighters under their watch.
For that matter, the F-15 has a perfect air-to air combat record, and would do well as an american UU - although it has never faced SU-27 flankers, the biggest threats it faced were mig 23/27's and Mig-29's
German UU should be a U boat too.
China could get a UU infantry/marine - reduced cost because they have billions of people to draft.
Heck, why not give every civ 2 or more UUs.
I'd have to go with Praetorians, I don't mess with rome Until gunpowder because of them, unless I can rush before they get Iron.