These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Broken Base: There's quite a bit of debate regarding which of the two games is better. The general consensus, however, seems to be that Ultima Underworld 1 is the superior game.
Crowning Music of Awesome: The second game's soundtrack consists of a rather excellent set of variations on a Phrygian progression.
The first game had surprisingly atmospheric music that was especially exceptional at the time of its release.
Game Breaker: By reducing a potion or scroll to a pile of debris, the player is rewarded with a clump of magical dust capable of endlessly casting the spell associated with the former item and never consuming itself. Daylight spells, potent heals, flying, and even Time Stop are possible. While it took a little effort in The Stygian Abyss (namely, hurling fireballs at said potion or scroll until destroyed), in Labyrinth of Worlds, the player could simply toss the potion against the wall until it became infinite arcane debris.
One of the Plot Coupons from the first game, the Sword of Justice, is fairly easy to discover on the third floor of the Abyss (you are, in fact, required to find it to complete the game). Once you get it, it's equivalent in power to the second strongest weapon in the game and has the added benefit of never needing repair. This functionally breaks the admittedly varied weapon tree right until the end of the game, when you have to destroy the sword to continue.
In the first game, it's possible to get your bartering skill so high that merchants will gleefully buy that one gold coin off you for this stack of identical gold coins.