I personally find Aztec UU pretty useless...its good that it doesn't require any resources but what's good of a swordsman with a strength of 5?
Honorable mention also goes to Gallic warrior, khmer elephants, musketeer and oromo warrior
i mentioned musk and oromo because i almost always skip musketman and go straight for rifling.
I also think ship UUs are not that good either.
I used to think the Aztec UU was useless but it isn't that bad. Admittedly I prefer a Chariot or Axe rush to Swords (except for Rome). Monty can get awesome Supermedics with 26 XP. The AI often can't handle two move units (keep to the Forests and Jungles). They can also execute nice chokes and worker stealing.
Musketeer isn't bad if you get gunpowder early. I also go quick for rifling usually. Oromos are a pretty good UU although you might delay rifling slightly but getting Drill IV rifles on 5 XP is pretty nice. I'd actually rank Oromos above average.
Some UU's and UB's you have to vary what you do with other civs. I don't normally sword rush unless I'm Rome and I'll delay Economics if I'm Spain.
Portugal's UU ranges from adequate to useless depending on the map. This doesn't apply to the East Indiaman - it is awesome on the right map. You can really set back an enemy Civ on a water map with that unit. Between Astronomy and Chemistry you have a tremendous advantage so Dutch will often go for fast Astronomy.