Realism overhaul is the best
I've sent probes in orbit around Neptune, Europa, Ganymede. Flew by Saturn. Sent a lander on Venus.
Sent a couple rovers on Mars. One of them climbed Olympus Mons (400 km ride).
I did a manned moon landing fully in first person and returned (that was extremely fun, thanks rasterpropmonitor and TrackIR).
I'm currently in the process of a manned Phobos mission with a centrifuge, with an explorer vehicle and a rover in Valles Marineris that has a sample return rocket to be intercepted by the main phobos spacecraft.
I did a manned return mars landing mission in RSS a while ago, but not RO yet. I had TAC life support, deadly reentry, FAR etc but I was using "cheaty" stock ion engines though. Looking forward to do that in RO soon.
If you guys are interested I can post some screenshots when Im back home