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.
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.)
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.