Well in the beginning of my career i had a few instances, where i had to dock without RCS or enough battery for SAS. That was quite a challenge (especially when combined with inexperience with docking in general). I do however believe that the most challenging docking i ever did was when my space station was moving around like a boat in high seas. It kinda reminded me of air-to-air refueling in DCS Fortunately i managed to catch it on video :) The challenge