I have to start with one of the most challenging talks ever:
Other talks of interest cover a wide range of topics.
Our Loss of Wisdom - Barry Schwartz
A Defence of idealism by Viktor Frankl
Gamification - how it can benefit. - Jane McGonical
A very different take on the voices in our head - Eleanor Longden
Of course I love Steven Pinker - he has several talks but this one explores our language.