I would also like to amend my candidate for worst UB. Dun takes it someone else pointed out the nichey-ness of it and I have to say I agree with that.
Cities settled on hills are not rare, and dun gives you a garbage extra promo on those units that makes it really annoying to attack them. I'd much rather a UB that has "reasonable chance to be useful and some chance to be useless" over "virtually always useless" when it comes to its ability to impact a game's outcome.
It has offensive uses too, but those are more commonly seen in MP since humans can't afford 5 defenders/city and still have a stack.
Worst UB: Assembly Plant ----> Resource dependent, marginal bonus over the basic building, and excruciatingly late despite the marginal benefit. Even the mall is better; at least it boosts wall street city for late game corps and allows you to get away with unorthodox civics with the bonus

. Despite that the mall is terrible, assembly plant wins.
(dis)Honorable mention: Mall, Feitoria, Forum
Best UB: Sac Altar ----> This is a really tough call for me, because there are 2 others that are also top-notch options. However, sac altar literally lets you get away with more/faster whip cycles (IE more production) and comes with a discount on top of that. It never obsoletes! If you can work slavery into the mid-late game without devastating

, you can even kremlin-whip off bio farms. Sick.
Honorable mention: Ikhanda, Terrace
Worst UU: Panzer. Outside of late-game multiplayer where nukes are banned (because if they aren't, they beat everything anyway and anyone sensible is going fission/rocketry), the threat level from AI tanks is virtually nil. Against anything else, even AI infantry/anti-tank/SAM spam, they're just tanks, and that comes with all of the disadvantages normally associated with industrialism + combustion. A very marginal boost on the very latest unique unit in the game? No thanks.
(dis)Honorable mention: Ballista Elephant, SEAL, Landcrap
Best UU: Quecha (single player) and Fast worker (multiplayer). Quecha is obnoxious beyond AI capacity to handle, successfully executing outright kills and chokes. It is the only civ banned in HoF gauntlets/challenger games because if it wasn't everyone would use it. It also takes out barbs with ease and even adds cherries by being available after hooking up metal for cheap HR garrison.
In MP, only very weak players will die or be seriously hindered by quecha attacks/chokes. After all, unlike the AI the human can simply use cheap warriors to block it from gaining defensive positioning in their territory, and then easily kill them. They do stop malicheesechokes and at a minimum keep people honest, but they aren't nearly the force they are in SP. Fast worker takes over, as it quickly clears forests after setting key tile improvements and the player needs less workers overall to set up. A faster growth curve against humans is very stout.
Honorable mention (SP): War chariot, Skirmisher, Immortal, Keshik
Honorable mention (MP): Skirmisher, numidian cavalry, dog soldier, war chariot
Most overrated UUs: Prat, Redcoat, Landcrap
Most underrated UUs: Jaguar, Cho Ko Nu, Keshik
Most overrated UBs: Forum, Dike, Rathaus
Most underrated UBs: Ikhanda, Sac Altar (these should always be in people's top 3)
Forum is overrated because not enough people consider it for bottom-tier. Rathaus and Dike are overrated because they're widely regarded as top UBs instead of simply good ones.
Prats are slow and not cost effective against axes; hard to use against competent humans or high level AIs. Redcoats are nice draft-able units but still need siege or spy help vs other rifles and rarely encounter something other than rifles that they can 1) outperform rifles and 2) beat. Landcraps are just liked too much and are bottom-feeder UU material, right next to ballista elephants.
Keshik terrain move goes beyond what AIs can handle. Jaguars can do a number of things that swords can not do:
1. They are cheaper in

2. They can get stronger-than-medic healing at 5 xp
3. They can fork and choke at 3 xp and will beat axes straight up on those tiles...badly
4. They require no resources to produce
They're not in the top-tier UU list with war chariot/immortal/quecha/fast worker/etc, but they're in the tier right behind. That they're even considered among bottom units is pretty silly.
CKN are serious business. They can be bulbed quickly, hold trait synergy, have the best pre-cannon collateral in the game, and defend obnoxiously well for an offensive unit. Mounted gives them trouble on the flats, but offensively against the AI these things are devastating. Same tier as the jaguar.