Yeah I was going for just the big ones but:
Grimy's Loot Mod for sure. It has the added benefit of continuing to give your scientists something to do, especially in LW2 where most of the high end research costs supplies to start so you'll often be unable to pursue new, core research. So far I haven't been blown away by what's in the crates but I've only opened the basic ones so far. I did just get armor with some special properties though, which is huge because other than the DLC armors I don't think there's anything like it anywhere else in game. Must have taken some serious magicks to get that to happen.
More Maps.
Even More Maps!
Even More PCPs.
More Cities.
World Expansion Project: Advent.
Dragon's Wilderness PCPs.
All good stuff for increasing map variety. (Although some of them can get huge, so without True Concealment some can cause you timing problems.) Definitely increases load times for some maps, you can easily tell which ones are vanilla maps and which are mod maps just based on how long it takes to load the mission.
Free Camera Rotation is a QoL must in my book as well. Can occasionally lock up the game when you're looking straight down and a reaction fires off on someone though.
Grimy's Loot Mod (+PCS and Alien Rulers Expansions), + Mod Everything + Customizable Secondary Weapons + Moddable DLC Weapons
I've got all of these and they're good. Doesn't make a huge difference but being able to, say, increase the armor shredding of grenade shots with a mod is pretty nice. Plenty of melee weapon mods out there too that can make your Ranger/Shinobis beast as hell.