Hands down, dedicated students...
I have seen teams with amazing mentors and great coaches that have done far below their level, not because the robot was not good or the robot was not robust but because the students dont seem to carry as much dedication towards it, I am sure there are a few but when a larger % of students dont follow the team its tough.
Not only that but these teams become harder to sustain, not as many people bringing in sponsorship or fundraising and not as many people traveling etc.
A lot of the mentors you will see are driven by the students, by seeing the students enjoy and learn from the process. Their dedication make us want to do it that much more.
I have been with 2016 for 5 years now and every year a new batch of kids comes in and find their way through the maze of a team we have... But they are there every night, every weekend for all 6 weeks. Thats what makes us want to be there. yea we enjoy building a robot and all, but if there are no students there to drive the robot theres no use having a good robot.

love having our students around.. and especially getting them to challenge the mentors idea's thats when it gets really fun!!