Is a "big ego" necessary for a tenure-track faculty position? I have worked in a few high profile research institutes for my PhD and postdoc, and find myself a little intimidated by the "strong opinions" and arguments that seem to crop up at the principal investigator level.  My personality is somewhat more laid back or agreeable, and I find myself wondering if this would be seen as a "personality flaw" in the somewhat cutthroat funding and scientific world (or at least that part I am exposed to).  I'm sure it helps to have a strong opinion of one's own work, but I sometimes find it hard to do so, as I tend to be more self-critical and more concerned about stating something correctly and figuring out what is "right" than necessarily being self-promoting and overconfident in my own abilities, but I feel like that ends up manifesting as some sort of fundamental flaw in relation to others I see advancing into the faculty ranks.
I think a "big ego", or at least a healthy dose of self confidence, is necessary for survival as an academic. Our jobs are one of repeated failure. Success rates on grants, publications, experiments, and job applications are often less than 10%. In the face of such frequent failures an abundance of confidence is a require for maintaining ones sanity. Further, many departments verge on dysfunctional and there is generally someone looking to push you down to get ahead (I am not saying everyone is out to get you, but there is generally at least one person in every department looking to get ahead at your expense.) Being confident, outspoken and to an extent self prompting is useful for dealing with these people. The difficult part is not being too self confident and too outspoken and being able to admit when you are wrong.