I would have to say that TDM Modpack should be among the first you try. You should learn the basic game at least a little before going off with an involved mod. TDM only enhances and adds new races so none of the other aspects of the game are changed under this mod. Basically it is like playing the unmodded game with more of a challenge against the AI.
TDM to start for base games addes a lot of challenge
FQM and any mod that use it for expanded systems and maps.
Pirates & Nomads for a good romp
Proportions for a slower more indepth game
Adamant (Under Development) for a challenge
AST for joyous journey to the undiscovered country
Star Trek Mod to go Boldly Where No Gamer has gone before.
And many many more.
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Creator of the Star Trek Mod - AST Mod - 78 Ship Sets - Conquest Mod - Atrocities Star Wars Mod - Galaxy Reborn Mod - and Subterfuge Mod.
AIC (AI compain by JLS) is a great mod too. Toughest AI yet because of specialised AI tech.
Just remember to use latest Versions of all mods !
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It is forbidden to kill; therefore all murderers are punished unless they kill in large numbers and to the sound of trumpets. - Voltaire