Worst AIs(in no particular order)
Isabella- She is ALWAYS, ALWAYS backwards in my games, probably because she is such a fanatical leader. She seems difficult to deal with, and I think it extends to the AI. She always seems to make powerful enemies who beat her down.
Montezuma- Monty is always backwards, though he'll destroy you if you neglect your military. Usually easily contained by telling him to go attack some one else and then cleaning him up once his out-dated army gets pwned.
Genghis- He never seems to do ANYTHING, and actually has me thinking he's a cream puff, the way I've abused him diplomatically.
Tokugawa- He never makes any friends, unless a religion randomly spreads into one of his cities. Most of the time, he is a backward and pathetically weak prick.
Saladin: See Izzie. Never, ever seems to be very powerful and has never amounted to anything but a backward zealot in my games.
Worst Leaders(IMO, no order)
Monty- I'm sorry, but the Jaguar is flat-out terrible. It loses one strength for....not needing iron, being 5 hammers cheaper than a swordsman, and getting +20% jungle defense. Ummm...wow. Axemen are pretty much better in every way, as +10% city attack means even less at 5 strength than it does at 6. And his traits have some potential too....
Mao- Philo/Organized? Where in the world is the synergy there? He's a frankenstein leader. They just don't match in any meaningful way that I can make out. The cho-ko-nu's not bad though.
Alexander- See above, except Agg/Phi. Another trait mis-match, and the UU is nothing to get excited over either, though it's quite a bit removed from the jaguar and the camel archers.
Saladin- His traits really aren't bad, but they aren't great either. The Camel Archers are really what gets him here.
Worst UUs(again, in no particular order and IMO)
Camel Archers- They don't require iron and horses. Yay. And..yeah, that's about it, unless they get some first strikes, which would make sense because they're supposed to be, y'know, archers.
Jaguar- In all but the most dire circumstances, they're the only UU that's actually worse than the unit they replace. And they don't even have a chance against axemen, and aren't even good at taking cities, which is what they're supposed to be good for!
Musketeers- I like both Frenchmen, but the Musketeers could use a little oomph, considering they obsolete faster than you can say "Knights." Did I mention that they don't stack up well against knights? I think it lands here more because the musketmen just plain suck than not being an improvement. The advantage just isn't enough to salvage muskets and their short window of usefulness.
Panzers- I'm correct in believing that the only way they're better is that they get a bonus against other tanks, right? Great. If I'm still playing at this point, I'm probably not looking for domination or conquest and am too busy powering towards a spaceship to fight my wars myself. That's what my lapdogs are for. All bets are off it's something else. I always have some gunships around pillaging to take care of tanks, but I simply aren't looking to polish off a big opponent at this point, just some backward weakling who I haven't found the time to finish off yet. If they get something more, all bets are off.
There you have it. My no doubt uneducated opinion....