Best: Orruk Warclans - A huge range of depth in its options and unit choices, seamlessly merges two entire factions into one cohesive force, and lifts the orruks into the top echelons of the competitive scene without being massively busted (some might say a little too strong, but that can be easily fixed). I don't play orruks, but every time I look at the variety of tribes and list-building opportunities, it gets me excited. Worst: Beasts of Chaos - Somehow this book kept me from playing beastmen, despite loving them since the early days of Fantasy AND already having a sizable army. Even when it came out, most units underperformed from lackluster abilities and warscrolls. Despite being the "chaos monster" faction, had some of the worst monsters in the game. On top of all of that, the lore constantly reminds you how despicable it is for beasts to align themselves with any one god, but then forces you into taking marked armies because they're simply superior. To this day it's still languishing near the bottom, and even die-hard beast lords rarely bring out their toys. I honestly don't know what they could do to even fix them at this point, which is part of why they dive beyond even the legendarily miserable experience of Hedonites. Good: Fyreslayers, Cities, Sylvaneth, Slaves, Khorne Okay, but problematic: DoK, Idoneth, Nighthaunt, Legions, Skaven, Ossiarch, FEC Poor: Hedonites - Shouldn't have left anyone's desk with how completely broken the base mechanics of this faction were, and the fact that players were shelving their armies in protest goes a long way to illuminate how poorly thought out this thing was. At least it can theoretically be fixed in FAQ (and maybe already has). Most Disappointing: Mawtribes - Though I won't deny the power boost the Ogors received as a result of this book, there were simply too many missed opportunities exhibited at every turn of the page. Cool rules addendum'd to be useless. Unnecessary restrictions on just about every trait or artifact. Only 2/6 tribes are playable (maybe 3 if you have a zillion ironblasters lying around). About half of the warscrolls felt like they weren't even looked at and copy/pasted over from 1st edition and will never see the table in any significant number. Nerfs to things that didn't need them. You're mostly pigeonholed into a handful of list types, with little way to experiment beyond that. I love the Ogors, and Mawtribes are in a decent place power-wise, all things considered, but when you look at how much of the book you're really ignoring to get that power, it can't be anything but disappointing. The book I'd give the "most squandered potential" award to.
Favorite: Gloomspite Gitz - A smorgasbord of units, battalions, and army compositions that essentially combine 3 small factions into one. There are so many ways to build this army, and it feels like you could spend years expanding and trying new things. Its only downsides are its lack of competitiveness in certain areas (namely anything that isn't hordes of grots), overpriced units, hyper-specific battalions, and some lackluster random mechanics. But even with these foibles, the spirit of the faction shines through and can make for some brilliant games of AoS (so long as you aren't going full 120 greenies).