Two-thirds of the way up the hill which eventually ends in El Alto, you can find the viewing point Andina Jach'a Kollo. Just don't trust your map or taxi driver to get you there.
"Why are we walking?! There are buses which go up to Killi Killi". Aw man, don't be a such a wimp, Jürgen! We need the exercise. And it's not even all that high.
Basically, any spot in La Paz can be used a makeshift viewing point. Just raise your eyes off the ground and there's the massive Illimani Mountain towering over the southeast. Looking up towards the west provides a view of El Alto, Bolivia's fastest growing city, stretching haphazardly across the cliff. And if you drive into El Alto, the panorama of the city below is unmatched. Though La Paz itself can be grimy, polluted and uninspiring from an architectural standpoint, I doubt any other city in the world provides as many incredible picture-taking opportunities.
The best bird's-eye view of Bolivia's capital can be found at the top of the Recoleta hill. The climb is arduous, but worth the effort. At sunset, the "White City" is even more beautiful from above than from street-level. Just head due south from the city center. As long as you're going uphill, you're on the right path.