I'd say the task I have in front of me is the interplanetary missions, I have yet to succeed on getting the object to the destination without running out of fuel and skidding right past to be lost. Would be a fun challenge to go rescue Jeb as I didn't think about it at the time, but due to its closer orbit the Moho late orbit adjustments are too inefficient in fuel and I burned it all there to readjust to make an encounter. What has me worried is that I had an encounter leaving from Kerbin but somewhere along the way it altered and I had to do adjustment burns. I am challenging myself to running multiple missions at once and colonize the system so I'm not accelerating time very much to make those journeys. So without a doubt sending craft to Eve and Moho are the hardest thing for me.