Generally, I would suggest to choose according to personal likings, taste, simple: the "chemistry" between you and watch style. What is the use of buying an expensive watch if you don't like it? Only, because somebody considers it as typical for a certain company? So if you like repeaters, buy not five, but only two watches and give yourself a repeaters and another one.
As a consequence, such a "Top 5 under 25k" list can but mirror my only, individual and very personal preferences and thoughts. Following this disclaimer, here is my list, respecting the 25k-dictate, in alphabetical order, with no particular ranking:
1. Acqua Perpetual
: Now discontinued, but the only way to purchase a UN perpetual calendar under 25k. A UN collection without the best perpetual mechanism in the world
2. Anniversary 160 (pink gold)
: UN's current milestone piece with the new cal. 160 movement;
3. GMT Big Date (pink gold)
: The unique and most practical GMT mechanism, a UN/Oechslin patent, deserves a place in the list. Since the Acqua Perpetual comes without the GMT (remember: < 25k!), you need to spend a list place for the GMT Big Date. My personal favourite: the 40mm model in pink gold, with silver dial; a classic watch. The more conspicious alternative: 42mm small second in pink gold, with silver sunray pattern dial.
4. Marine series
: No watch design is more identified with Ulysse Nardin than the marine chronometer style, one of the Marine series definitely has to be on the list! There is ample choice, but I would suggest one of the more classical ones, so no diving watch and no chronograph. You can get away relatively inexpensive with the first and most famous of UN's Marine series, the 1846 Marine Chronometer (38mm) in yellow gold. the difference to 25k could be spent on another watch. If you decide to exploit the full price limit, then you could take it with gold bracelet, but this would not be my choice. The new Anniversary Marine Chronometer with white lacquered dial is over 25k, although only barely. Both styles, the Roman numerals and the Arabic numerals, are genuine, but I prefer the Roman numerals on white lacquer as the more chronometer-like, over the various dial styles of the Maxi Marine Chronometer.
5. San Marco Chronometer, steel, with blue enamel dial
: Also one of the UN classics. I would have chosen one of the cloisonné enamel dials, but these are over 25k. Yet the plain blue, transparent enamel on the silver guilloche base is worthy a list place on its own.
You see, all my choices absolutely respect the 25k limit.
If I were to choose personally, I would maybe limit my list to three watches only, distributing the total sum of 125k on these three:
1. Perpetual GMT 38mm, pink gold, smoke dial
: For me simply the most desirable UN that is perfect for everyday wear: By combinating both, perpetual and GMT mechanisms, you can cleverly save one list place.
2. Marine Chronometer 1846, yellow gold, white lacquer dial, Roman numerals
3. One these three, all of them over 40k, but since you save a lot with the 1846, this is justified:
a) Hourstriker, pink gold, black enamel dial
: Combination of three classic UN techniques: striking, Jacquemart and enamel, my favourite
b) Freak 28,800, white gold
: I would have preferred the Freak 1 in white gold, but this is not available any more; however, the Freak 2 is a milestone on its own.
c) Sonata, white gold, silver dial
: Also a milestone, and after the Freak 1 the first fully self-developed and produced UN movement.
What, no Anniversary 160? No, because a) it uses the Dual Ulysse escapement that is used already in the Freak 2, so by taking the latter, you have the escapement, and
b) the cal. 160 is going to be used in a new GMT Big Date, so waiting for this allows you to have two original UN complications in one watch.