One of my favorite mods, when I disable half of its features in the configs.

There are features in Quark that are downright difficult to play Minecraft without, and really ought to be added into vanilla as soon as possible.
Linking items in chat is practically required for playing multiplayer, the option to build on the far side of blocks by sneaking is a life saver, Glass Item Frames are the decoration block of the century, not to mention, all item frames work on the top and bottom of blocks, the tooltips for food and armor have information that is extremely useful and cleanly presented, and I personally end up making a lot of use of the emotes and fairy circles.
However, not every feature is on this level. The biggest standout are the ghosts, which are three white squares that make random sounds, rather than having any of their own, and just kind of spawn around in the nether. The autojump keybind is exceedingly annoying, and I'm very doubtful that anyone in history has pressed it on purpose. The animal texture variants are hideous, Quark somehow managed to make a frog that I don't like, and that takes effort, the Nether is full of normal melee monsters, which isn't what the Nether is designed for, guardians free-spawning at random in the ocean is an unhinged idea, axes fast-breaking leaves instead of hoes leads to you accidentally wasting tons of axe durability trying to mine wood, and the vast majority of content comes in the form of mid-tier Building blocks that you won't use, because you're playing modded, and there is no chance your other mods don't have better options.
For a mod based around customizability, Quark is remarkably bad at having the kinds of options I'd want. For instance, I don't like the normal animal variants, but I love the idea of rare shiny variants of mobs, but they cannot be enabled separately, you get one, you get both. The Tetris enchanting system is really cool, but other mods rely on enchanting to work the way it does by default, I'd love to just have a variant of the enchanting table with a different crafting recipe that does the tetris enchanting, but instead you cannot enable it without removing default enchanting.

Overall, as stated in the title, one of the most essential mods out there that I never want to play without, but configuration feels less like tweaking it to my preference than hitting it with a hammer until it behaves.