If you want to have the real Stars Wars' watching experience, then you should watch first the episodes 4 through 6. Later, when you have some time lose and you don't know what else to do, then you can watch the episodes 1 to 3.
Even if you plan to watch the full serie 1 to 6 in a single swoop, you should begin with the episode 4 to 6 first; as this in my opinion the only way to have the real experience because they have been made in that order and therefore, you will be able to see the evolution of the serie as made by the directors.
If you look first at the episodes 1 to 3, non only many surprise elements of the classic trilogy will be spoiled but chance are that you will get tired of some of its more interesting features before even beginning to watch the real deal. In short, you can consider watching these three episodes first as a long, big spoiler for the next three.
As for the animated series, watch them last, if you care.