To me Topcoder is a melting pot where talented people from all over the world gather. It is an awesome place to be!
I'm not quite sure what qualifies as 'greatest', because each contest has its own unique challenges. There is, however, one particular contest that I was extra excited about. It was the Asteroid Data Hunter Challenge.
You see, my childhood dream was to become an astronaut (lol). So, winning the challenge made me feel like I accomplished a little piece of that dream.
Playing video games and reading manga 😀
Pay close attention to the challenge forum and client feedback. They always contain crucial information and will greatly improve your work.
I found out about Topcoder from a friend in my last year of college. Actually at first, I joined Topcoder to participate in the SRMs. After a while, I started poking around in different types of competitions and stumbled upon the wireframes. Needless to say, I got hooked and have been in the wireframe squad ever since.