Well, you know my difficulty...Emperor and starting to get there gradually on immortal. Used to play epic exclusively but now I'm trying everything but marathon (can't...take...the...slow...games...)
I know how you feel. Marathon takes far too long for my taste.
Some of my "top tier" UBs:
Ikhanda - this is money for an early rush strategy. More expensive than a normal barracks but aggressive mitigates that. Shaka can recover might fast.
Indeed an awesome UB with lots of synergy with leader and civ!
Sac. Alter - I like it for the same reason you do. Whips can be applied liberally...even 1 pop whips sometimes!
If some of you reading this thread has managed to avoid my outbursts of dismay against slavery, I'll sum it up for you. I hate the civic. So I cannot agree here. Perhaps in order to learn to love the whip (sounds wrong...) I need to play with Monty some more.
Hammam - I'm a fan of early

. UBs that grant the ability to do this while taking a different (and in many cases stronger) tech path are very welcome.
Indeed! This is an incredible UB, and I ususally leverage the early

by going for Trebs/Knights/Janissaries faster instead of wasting time on happiness.
Terrace - Every city gets a granary. Every city is up to speed quicker, saving hammers on a monument. I mean, everyone knew HC was weak so they went and gave him a good UB at least.................

.
... I don't like the terrace. Sure, it's nice and stuff, but frankly, since I am not a whipper, I don't use granaries as much as some of you others, and I think it's stupid to build the terrace only for the culture when it is more expensive than the Monument. Plus I don't play Inca if I can avoid it. I'm not especially into their traits or uniques although I understand why many people like them.
Ziggurat - I am SHOCKED to not see this appear as of when I'm typing this in this thread. Its REX synergy is outrageous. Courthouses at PRIESTHOOD?!
And is cheaper. What more to say (actually an ORG/AGG leader of Sumeria would be a blast!)?
Honorable mentions:
Cothon - Yeah, the TR's can be pretty nice...
Agree, ofcourse
Dike - I don't put them top tier because they come too late IMO. Very helpful on some maps but remember a lot of cities don't want to be working water tiles as they have better options. Still a sound UB with lots of use potential though.
Agree completely here aswell.
Rathaus - Overrated IMO, but still good.
m-hm - agree.
Obelisk - Only recently have I learned how clutch early GPP are. This early enough? The obelisk gives the player some pretty unique options.
Best utilization I've done of a UB; Funding an early war based on food and hammers/gold from priests to chariotrush - incidentally my neighboor was Izzy so I had a use for the prophets

.
Ball Court - If you want a catapult war, this thing is king.
And I am the type of player who almost always wants a catapult war. I don't fancy the rest of the Mayan package though so I don't get to use this as often as one might want.
Madrassa - Another building that grants specialists. GP farm powers up.
I only recently tried this out and it is awesome, so I agree with you here. Sal turns into quite the cultural/economic beast when founding religions and using the priests from this one
plus shrines from the GPs.
Hard to say which are powerful, alot are game specific and some work better with certain leaders. However, here are a few I tend to prefer
1) Dike. Willem is financial, so post dike sea tile get 2 food, 1 hammer, 3 commerce. THAT is pretty big.
Why yes but I feel it often comes too late to decide a game. If I reach SP with the dutch the game is in the box. If one of my opponents does, I am probably going to lose anyway (especially since Willems AI is one of the better).
2) Ikahnda: Half price barracks because of aggressive yet works very well with a large empire with the expansive trait to cut down costs and grow cities larger faster because of half price graneries.
Agree, as stated before.
3) Forum: Only Agustus. Industrious leader with a UB that get +25% GP. Add the Parthenon and he has close to that IND/PHIL taboo trait combo. Makes him the best wonderspammer (yeah, better than Ramesses).
I disagree. I like Julius better, using merchants to fund the empire your praetorians get you.
4) ODeon: Mostly Perciles since they are half priced. Extra happiness, extra artist slots. A powerful UB for a cultural win. And it improves late game with the extra happiness from the late game wonders.
agree, extremely good UB aswell.
5) Assembly Plant: Fred mostly since they are built 4 times faster if you have coal. The extra engineers work very well with Fred's Philosophical trait.

Too late for my taste but I do catch the thing about them coming up so fast.
6) Salon: Yeah probably an odd one, but a free artist while adding a +25% beaker building is big if running representation which I usually do as the French.
Yeah. Because playing french means playing either for culture (Louis or De Gaulle) or total war (where happiness and science matters - Rep will be used).
7) Stock Exchange: With Liz and Vicky. Financial with a +65% gold building? That's big!
I like all the english leaders but apart from that, Agree.
8) Research Insitute: Mostly Peter and his Philosphical trait. Every city get's a Great Library! One of the very few UB's that really help in the space race.

If you reach that part of the game without achieving Domi/Conq as Russia (Cathy

), settle down for your fastest spacerace vic ever.
9) Madraasa: Sorry HC lovers, this tops it. Comes almost at the same time, adds +2 culture, and opens up 2 priests. A combo of the Incan Terrace and the Egyptian Obelisk rolled into one!
Already commented
10) GER: Not many UBs give military help (Sitting Bull, Celts, Ragnar) but this adds an extra 2 XP to mounted units! Great with Genghis and theocracy as you start with a 9 XP mounted unit which is 1 XP from getting another promotion and thus pumping up the chances of faster GGs!
I find that this 9xp-thing fits extremely well with the use of Keshiks - they are often used in a way where getting that healing from the third promo is worth it in itself. You rush with them, you don't stick around for five turns with a medic. A medic for one turn and the promo on and we go again.
How do you figure? Unless you need the happy, or have at least 4

a forge will do nothing for a commerce city, unless its a mint.
How do you intend to build Markets, Banks, Grocers, Libraries, Universities and Observatories without hammers? If you are not building these in your commerce city you could just as well pour half of the commerce away.