Up until recently, I'd have argued that Settra the Imperishable and Necropolis Knights were the two most powerful units available to the Undead, both extremely strong and flexible, and both hideously underpriced in points for what they bring to the table, especially in combination with each other. And from what I hear this was borne out on the tournament scene, with 'Settra plus 2x6 Necroknights' being a rather obnoxiously common undead core.
Previews from the generals handbook 2 show that both of these units are getting dramatically more expensive, shifting them from 'outright broken' to 'slightly overpriced, but still powerful enough individually to be worth it if you don't run too much and keep the fat trimmed in the rest of your list.'
You'll no longer be able to field enough necropolis knights to overrun the entire table, so you'll have to choose your targets, weight a flank, and use other cheaper units to tie up the rest of the opposing army while your necroknights pick apart one target at a time.
Settra is harder to justify, as he's kind of fragile for his new points cost, but nothing else in the faction buffs like he does, so there's still some justification.
Otherwise, necromancers are amazing, large skeleton warrior blocks are very good, all of our monsters and monster-riding heroes are pretty strong, heralds are great if you're running multiwound deathrattle units (like chariots and necroknights), wight kings with black axes are solid generals in low points games, as are tomb kings with shields, especially if you're also running one or more heralds. Zombies aren't strong, but they let you skimp on battle line points and still mob up into a single usable larger block, which frees up a lot of flexibility in list construction. Spirit hosts are very good. Liche priests are alright. Screaming Skull catapults don't deal a lot of damage, but the bravery penalty is very nice with screams. Chariots are good if you're running tomb heralds. Corpse carts are great if you're running both zombies and some casters. Banshees are nice if you can inflict bravery debuffs. If you aren't running a named general like settra, ruler of the night and red thirst are both really strong options, as is the tomb blade for keeping a monster-riding melee hero in top form.
The only undead units I've run in AoS that I didn't like at all were Black Knights (their offense is just super disappointing, whoever wrote their rules seems to have thought they were mounted skeletons, when they're supposed to be mounted wights), the black coach (it's not awful, but it doesn't really contribute much either), the tomb-king-on-chariot (he's a tax for the chariot formation and not much else), and the casket of souls (way too short ranged for an immobile piece, doesn't hit very hard at that, and the casting boost only affects liche priests instead of all death casters like the mortis engine or corpse cart).
Most other stuff I've tried is... alright. Not bad exactly, but not especially impressive either. Like, I wouldn't exactly call morghasts bad, but even at the new increased points costs I'd take a trio of necroknights over a pair of morghasts every day. Same with most the other stuff I haven't mentioned. Grave & Tomb guard are alright for elite infantry, but they don't really fight all that much harder than the same points of regular skeleton warriors, which are battleline. Archers can be nice for the flexibility to apply wounds at range, but their punch-per-point is rather low. Etc.
Again, I haven't played much with or against Flesh Eaters, so I can't really identify any particular winners or losers there.