Since the criteria is "favorite", I pick Mongol. Very fun UU/UB combination to use.
Picking the "best" here is tough! Inca/Mali/Maya/Persia/Rome/Viking/Zulu/Dutch/India all have some candidacy for "best".
I'd personally say over a while cause no one responds to this.
Ever.
If you still think the Dutch UB is a top option at this point in time, I strongly suggest some time spent considering the returns of other UBs. The Dutch UB is very average. However, on water maps the UU can go crazy, especially with a fast astronomy bulb. 200 AD astro on a water-heavy map, when the best unit someone can put forth vs you for a long, long time is 4 str? Hello, blockades! Or amphibious raids. The majority of the Dutch fanboys don't even know how to play them optimally.
But those Dikes simply are the best building in the entire game for me, especially with Forges, Factories, and power. The only drawback is that they come a bit late.
And that they boost what is among your most marginal tiles, which if you have any health constraints at all you might not be working.
Seriously, all this jizz over the Dutch UB and it's probably inferior to the Aztec UB, on WATER maps!
Random Thought: What's the over/under on how long before this thread degenerates into a brawl over the relative merits/limitations of the Dutch UB?
You win.
Of all the civs on this list, Mali is the most chronically underrated. The UU is strong in SP and capable of rushing below deity easily enough...and doubles as an amazing defensive unit. In MP, the UU is complete cheese and screws non-inca or persia w/ horse badly. They fight at like 90% odds vs archers ---> pillage + choke party anyone? The Mali UB is so-so, but forges are relatively common at least and their function as

buildings with luxury resources makes them viable builds in commerce cities, so you will tend to benefit from it at least...and it isn't late either.
Maya is also very good, although their UU is less powerful in a rush since it takes longer to get and is more expensive...but their UB is quite awesome.