He is/was pretty eloquent behind a teleprompter. Not so much without one.
President Obama is a brilliant guy. His academic career alone at Harvard Law School settles that question. As far as speaking ability, you have plenty of examples of him speaking without notes or a teleprompter: any time he is interviewed on TV. If you watch those interviews you will invariably see a man who not only has a command of the issues, but knows how to phrase his answers in the best possible way.
Presidents have used Teleprompters since Eisenhower as a tool to assist when giving a speech not to mask ones ignorance. Without a teleprompter trump talks like a third grader who can barely string four words together to form a sentence. Trump just babbles incoherently. I remember what seems like ages ago, before the teleprompter meme or Teleprompter Derangement Syndrome, Republicans said Obama was suspiciously too well spoken and slick.