Cult of Slaanesh is a high quality list. Maybe slightly stronger than general DE, but general DE are pretty weak so that isn't a problem. It has variety, it is different, it encourages interesting games.
The Daemonic Legion is pretty well done, but might be just a bit too hard. Admittedly its great performances are likely linked to being played almost entirely by veterans, but its pretty damn hard on paper too. Certainly the Tzeentch flying circus is yet another exploitable loophole in list design.
Grimgor 'Ardboyz and Middenheim are okay lists, they just aren't very interesting. The new army books make them both largely obsolete.
Sylvannia is a decently designed list that fits in well at tournaments against general purpose lists. But its extremely easy to design a list to beat Sylvannia, so it isn't balanced.
Archaon's Horde looks okay on paper, but if you dig into it, the only real reason to ever field the list is multiple units of chosen knights. And multiple units of chosen knights is very rock-paper-scissors.
Edit: forgot Eshin originallyThe Eshin list is extremely annoying. The entire game is about the Master Assasin. Either he kills enough of the enemy and Eshin wins or he dies early and Eshin loses. Every game I've played against Eshin he inflicts the vast majority of the army's damage, sometimes he has inflicted literally ALL the damage. Warhammer should not be so much about one model.
The Errantry list is an overt mistake.
Lets give every unit a free 50 pt magic item! Yeah, that's a great idea.
Someone wasn't thinking.
Slayers and Sea Patrol are abominations and crimes against the game. The most one-dimensional and borings lists in the current game, by a lot. Doomseeker has to be the stupidest balance concept ever ... a 65 pt model that wipes out most infantry units by itself.