External conflict is fun. Bad guys resulting in a chase, hunts, explosions, all those things that happen in movies or Robert Ludlum books. They're fun, sometimes necessary, and when they take you to exotic locales, even better.
Internal conflicts are the best. Not contrived stuff like oh he isn't talking to me and then not talking to him thereby going around in a convoluted circle of silence that could be fixed by a good shout of "WTF?" No, I'm talking serious issues. Things that cause you to cringe because you're not sure if they'll ever get resolved.
Historicals are great for this, the conflict is inherent and the taboos more prevalent. I love that about them, though contemporaries are also great. The more things change and all...just because women have more rights now, more say in our own lives doesn't necessarily mean smooth sailing for relationships. In fact, it means choppy waters. A 2 career household is difficult to maintain unless compromise is key. Getting to that compromise can be ever so fun, however.