Official DLC Guide

Here is a list the official /4xg/ stance on Stellaris DLC as decided by myself

Utopia: Buy
Gives more ascension perks including ones to transfer your pops into robot bodies, mastering genetic engineering, and making your species psychic so they can dive into the Warp. It also adds megastructures which basically serve as an e-pen to show at how good you are at ecoing. Also added hiveminds.

Leviathans: Buy
Adds giant space monsters for midgame fun. Defeating them gives rewards like unique techs and resource rich systems. Also adds enclaves which you can purchase buffs, structures, leaders, or resources from (different enclaves have different offers). Also added War in Heaven for late game F U N.

Distant Stars: Buy
Adds more anomalies and events along with more leviathans. Adds special L-Gates which can lead you to an extragalactic star cluster with several different options waiting there.

Apocalypse: Buy on sale
Adds space mongols. They will demand resources or they will beat you up. You can hire mercenaries from them or pay to raid rivals. Chance of them going Genghis Khan on the galaxy in midgame which can be fun. You can now build titans as well, which can hit things across entire systems. Also adds planet destroying weapons but they are super expensive and not worth it. Only good thing they do is give you a nice casus belli. Mongol stuff can be fun or annoying, titans are neat but unnecessary.

Synthetic Dawn: Buy on sale if at all.
Adds robutts as playable species. You have to play them as a hivemind because Wiz is a dumb. Can play as Skynet, Borg, or rogue sexbots. Adds a robot fallen empire. Mediocre, robots are pretty similar to traditional empires aside from sexbots. If you don't care about robots don't buy.

Megacorp: Buy on sale if at all
Adds new empire type that is basically just a normal empire with some small differences. Adds some new megastructures and a new planet type that is pretty powerful. Added slave market and lootboxes. I don't like it and didn't buy it so that is all that matters.

Ancient Relics: Buy on sale
Adds archeology digs with rewards that are not balanced for multiplayer. Can get relics which turn purple mana into cool effects on top of passive boosts. Adds another new mana type. Two more precursor empires added. Not a lot of new content.

Plantoids + Humanoids: Cosmetic garbage
Adds some portraits and a new ship design each. Waste of cash.