What are your favorite mods?
Note that this question is touched upon here, but only for very large, expansive mods.
Also, I don't play a lot of games and have never installed a mod (and most mobile games don't allow it anyway and none on console) so this is mostly based on how much I like the concept of the mod and the footage I've seen of it, which is obviously to be taken with a grain of salt and is gonna be a lot different from a list from someone who actually plays the game with the mod. This list will also be biased towards well known mods and won't be expansive for the same reason.
KSP
Principia
Kerbal space program simplifies gravity by only doing calculations for one body generating gravity, meaning Lagrange points don't exist and orbits are generally simplified. This mod changes that, and not just for your ship, it does for the planets too.
RSS (Real Solar System) mod
Title. Basically it recreates the real solar system in Kerbal space program since KSP has it's own system with it's own planets. Usually followed by Realism Overhaul (a mod with includes real rockets because the KSP system is 10 times smaller for playability) and Realistic progression (a mod that makes all those rockets disponible in the game's tech tree and would probably be high on this list.)
Outer planets mod
Basically a mod that adds Saturn, Uranus and Neptune analogues to the game, and some of their associated satellites, while still being relatively original. (note that they do add a pluto analogue but the stock game already has a pluto analogue. It's turned into an Enceladus analogue around the Saturn analogue.)
Honorable mentions:
10x Kerbol system, because it scales up all the celestial bodies in the game to a realistic size without needing to change the rest of the game.
Tilt' em!, for adding axial tilt into the game, something that is moderately important to orbital mechanics (if your planet is inclined relative to the rest of the solar system, your orbit might need to align with the solar system instead of the planet to reduce inclination in interplanetary space.
OptiFine for Minecraft. Most shaders cranked up to max will roast the beefiest of computers, mind you.
i heard sodium mod is better nowadays.
I'd argue they're not really directly comparable. Sodium is in the Fabric modloader ecosystem while Optifine runs standalone or alongside Forge. So in practice you'll never be making the choice between one or the other.
I am a fan of jellyfin's work though, including their other performance mods.
Computercraft for minecraft. its incredibly flexible and can do most things computers can do. I have a friend making a full network stack for it in lua. hes already at the transport layer.
also KSP
Near future suite, Kerbal atomics, Station parts expantion, Far future tech, and other neartea mods.
Nearta is an incredibly talented mod author who creates beautiful part packs (often with their own plugins for new and interesting mechanics) based on concepts for future missions/spacecraft. He has everything from gigantic solar panels in NF solar, to many more electrical engines in NF propulsion, more nuclear engines in kerbal atomics, or fusion engines in Far future technologies. Did I mention his models are out of this world gorgeous?
I'm a bit fan of Kaiserreich for Hearts of Iron 4 due to the sheer amount of content it adds. I love playing with the base game and real world but when basically none of the minor nations have focus trees and you have to pay £100 at least for all the content it doesn't even hold a candle to Kaiserreich as a mod with almost every nation having a tree and it's free!
A few of my favourite games started off as mods such as counterstrike and dota.
A mod I loved was a series of Starcraft 2 maps which iirc were called OBS.
These were basically 1v1 maps that allowed there to be 8 players who could then play a series of 1v1s with alternating players playing and spectating. In the early days of local tight-nit sc2 communities this was a great experience as it kept the communal spirit of the pc cafe meetups alive online in the gap periods and helped coach the new players.
Another mod template I always appreciate for RTS games is grid keys. This basically maps the hotkeys of a game to visually map to the grid on screen, allowing you to quickly pick up new games without relearning muscle memory for each games layout.