There's quite a few, actually.
Both Palkia and Dialga get access to Heal Block which, when using a Psychinium-Z, gives the user +2 Special Attack while also preventing the foe from healing itself, whether it's from Recover/Roost or Leftovers. This turns both of them in to powerful wallbreakers. Palkia, in particular, really needed this buff.
Arceus now has a different way of becoming any particular type it wants to be by holding any of the types of Z-Crystals in the game. For example, Arceus can become Groundceus by holding Groundinium-Z and can use the corresponding Z-move along with it. However, it can't utilize the 1.2x boost towards its STAB since you no longer hold the Earth plate, but the payoff that the Z move itself brings with its immense stopping power, especially after a Swords Dance, is more than worth it at times.
While not as common, Darkrai can use Z-Hypnosis (which is slightly more accurate than Dark Void now) to put the foe to sleep while giving itself +1 speed. Xurkitree, while not Uber, does this as well. This allows you to outpace nearly everything in the game and, granted you land the Hypnosis, give yourself a free Nasty Plot boost with Darkrai or Tail Glow with Xurkitree.
There's plenty more out there, such as Electrium-Z Solgaleo with Wild Charge to slam Ho-Oh, Waterceus and Primal Kyogre or Z-Splash to give Solgaleo +3 Attack, but there's so many different possibilities that it's simply not possible at the moment to list them all. We're still discovering new combinations and I'm sure we'll still see new Z-sets in the many months.