Favorite thing: Years ago, I was an intern at the 28th floor of the shining tower of this building. I stayed in this bank for 6 months and they even proposed me a contract !!! But, working in La Defense, from my point of view, is sharpely synonymous of despair and uniformity (in the metro, from monday to friday, you see that everybody wears the same suit, carries the same attache-case and, over all, prevents himself or herself from smiling) ... No, thanks !!!
Like DJ does, enjoy the panorama from the Mont Valerien.
You can have a comprehensive view of La Defense and of the West of Paris.
And it is free.