Not really coding, but debugging complex problems. I love it when I have to dive in head-first and dig (very) deep to find answers to super-complex problems. I once went into the internals of a programming language to understand why a library was acting up in a particular scenario. Another time I had to optimize and re-compile from source (after modifying it) so that the application would not leak its memory. (Of course, I contributed it back to the language).

The inner satisfaction that you get after all that hard-work when it finally works, pays off! Bliss!