The worst endings that professionals and students alike repeatedly employ are:
1. "That's it / I am done./ Thank you!"
Ending all of sudden in this way will not prepare the audience. At the same time it is neither powerful nor professional.
2. "Thank you for your time!"
Your gratitude should be communicated to the audience through your passion and delivery during the presentation. The audience will appreciate it better.
3. "Any questions?"
Asking questions at the end may not only go off topic, controversial and lengthy, it will also deflate your ending. If you have to take questions, do it during the presentation.
4. "Let me conclude by saying..."
The first reaction that the audience will have is: Here you go again! If you do have to summarize, do it in few points and with a beginning like: "What we covered was.. ", "The take-away for today is..." And do not forget to follow with a powerful ending.