Well… What a coincidence…
For Trident realms:
Fav unit is hard to pick, but yeah, Knuckers are right up there, precisely for the reasons @DarkBlack listed.
Always so satisfying when your opponent frets about a unit in your army that only has 6 attacks and no CS. It’s a unit that rewards smart play in such an incredible way, gotta love it!
But TR are blessed with so many good choices. I love the river guard (who doesn’t love murder frogs?), heartpiercers (the formation makes them stupidly good, I expect a nerve inbound) and off course the lovely Siren, so solves so many problems on her own! And naturally the Kraken, oh boy can that guy take a hit and keep on rolling!
For undead, fav unit:
Wraiths! Damn flying + surge + def6 is absolutely crazy!
But special shout out to Werewolves with brew of caterpillar, a flying pharaoh and Mummies, oh boy those guys just manage to get Shi*t done! Perhaps slowly, but they WILL get it done!
Most hated unit? Erhm that is a hard one, outside of spam lists I hardly ever feel that a specific unit is over powered.
Although I would probably go for the cliche, of the big expensive flying beasts. I know they are expensive, but I feel they don’t encourage smart/creative plays. This is naturally very subjective view, but I feel like they are tricky to play against, but often times very simple to run. I really like the flying monsters/characters that hit hard, but are rather fragile, because you kind off have to play careful with them. Where as these days there are these titan-flyers, that have high nerve, high defense and generally good CS. Which means they are generally okay vs almost anything.
Obviously this is counter acted by the fact that they cost a ton of points, so I wouldn’t say they are necessarily OP, it is just that they don’t invite/encourage smart plays, which I find rather disappointing on a flyer unit.