This is not an extensive list because I don't have thousands of hours in KSP.
Colony & Exploration
There are quite a lot of mods you can choose from.
Wild Blue Industries series of mods by Angle-125: Buffalo for rovers and ground exploration, DSEV for spaceships, Pathfinder handles colony building and resource harvesting, and Snacks for life support.
USI series of mods by Roverdude: MKS is the ultimate colonization mod, where maintaining a self-sufficient colony almost always requiring the setup of multiple outposts and transport logistics. Parts of this mod allow for "virtualized factories" so your game won't lag out. It plays very well with other mods within the USI series. If you are planning to use MKS, then I recommend stick with USI and do not install other life support or colonization mods.
Kerbal Planetary Base Systems: parts to build planetary bases. It does not feature any life support or other systems, but it plays well with almost all life support mods.
Stockalike Station Parts Expansion Redux: parts to build orbital stations and colony ships. It does not feature any life support or other systems, but it plays well with almost all life support mods.
TAC Life Support (TACLS): a standalone life support mod. Can be used with many other mods.
Interstellar
Near Future Technologies and Far Future Technologies: reactors for power generation, ion, fission, fusion and antimatter engines. A few well-known ones (e.g. Daedelus) are missing, but still very good series of mods. They also run surprisingly well and mostly expands and uses stock systems.
KSP Interstellar Extended: the most comprehensive interstellar mod. Reactors for power generation and propulsion, all kinds of crazy engines, photon sail, beamed power, and end-game warp drives. This mod is very complicated, performance heavy, introduces many new mechanics, and many tutorials are out-of-date, so be prepared to spend a lot of time researching and testing spaceship designs. Recommend start a sandbox save first and search on KerbalX for craft designs and Youtube for tutorials if you get stuck. This mod does not play well with Near Future Solar or Near Future Electric because they will nerf KSPI-E reactors into completely unusable.