Why would Putin want Trump as president? It's so simple that it doesn't really deserve writing up, but I have time to spare just now.
1. The proposed policies by Trump are very favourable to Russian interests. Reducing US NATO presence in both Europe and Asia in particular, as well as him suggesting he may lift the sanctions against Russia and acknowledge Crimea as part of Russia.
2. A Trump win would certainly weaken both the credibility of the US government and their relations to their allies in Europe and Asia, leaving space for Russia.
3. A Trump win would also be likely to break havoc in the US economy, again helping to increase Russian influence around the world.
4. American internal conflicts (some of which may take place now regardless of who wins), would also be an opportunity for increased Russian influence.
5. Putin may believe he'd be able to direct the overconfident fool in whatever direction he wants. That may or may not work, but even if it doesn't, the rest is still reason enough for Putin to cheer for Trump.