Steve, i'm using KSS and GPP combined, along with OPM. That was the best combination of multiple planet packs I could find in order to create the largest play space.
Along with that you grab Interstellar extended, the entire USI suite (kolonies, konstruction, life support etc), RemoteTech and then a big key is ResearchBodies +Tarsier telescopes so you won't be able to see any of them aside from the closest ones until you start discovering them.
I also took the governments + cash mods so I could be rewarded for things like having a station house many kerbals. Also extra planetary launchpads/Hangar modules for ships because it works well with the USI stuff and with such a large universe it helps to be able to launch in orbit or from established leapfrogging bases as you expand outward from Kerbin.
Aside from that, go ahead and add in any that will help in interstellar travel + whatever utilities interest you. Mech Engineer, MechJeb, Transfer Window etc (and for me, personally, ANY mod maintained by linuxgurugamer is going to be good.)
In my opinion that describes just about the most fun one could have in KSP via career. Going further, I'd disable anything cushy like reverting launches and such, while allowing yourself bit more tool use.