It would have to be the Borg. It would really all depend on who had the greatest desire to win. With a good stategy and plan, both of these true enemies can be defeated.
When Captain Picard charged the Enterprise E into Shinzons' ship, the Scimitar, that was amazing. He did not know what the repercussions would be other then disabling the powerful ship. Captain Picard stood and faced the enemy in the eye and attacked him head on even when the odds were against him. That is why I chose the Enterprise E destruction scene as the best.
I am sorry this is about 6 months late. As it seems, there are only two of us left from our class. The funny thing is that the people who were the biggest pain in the butts did not last long. Too bad. The past year has been great. I look forward to the next.