I'm going to rephrase this as "mods I wouldn't consider using Galacticraft without" or "mods without which Galacticraft space travel would be too difficult or annoying":
Any industrial mod for compatible power, per personal preference (Big Reactors, Atomic Science, etc.)
Modular Powersuits
NEI. (few mods are simple enough to not benefit from NEI, and GC is no exception).
Thermal Expansion (for Tesseracts, almost exclusively - power and, if I ever get GC 3.0, liquid O2 generation off-world is so nice, in whichever direction you prefer. Put your bulky reactors on the moon where they're out of the way, or just keep your moonbase efficient and compact by powering it from the overworld, etc. In GC 2.x my off-world bases are pretty much one small room attached to an enormous O2 plant, which is kind of annoying.)
Mystcraft (I know the rockets are half the fun, but getting back and forth is much better with this mod)
Any minimap, per personal preference, especially one with waypoints
Extra Utilities (almost entirely for Magnum Torches - empty dunes that are nothing but infinite mob spawners plus month-long nights on the Moon are intolerable with normal mob spawning around your base, sorry)
Carpenters Blocks (space bases just don't look right without angles - this one is only partially compatible, though. Any non-Vanilla transparent block IDs you add to the config will be sealable on their own, but NOT as textures for Carpenter's Blocks. Blocks that are sealable by default work just fine as CB textures, though.)
Any microblock mod you like, per personal preference. I prefer Immibis' Microblocks.