Rachel Glennester is excellent. In so far as I know them, I'd trust her views on the effectiveness of development policy. She did great things as the UK Chief Economist for the Development department. She is a GWWC member and has talked at an ea conference in the. I'm not sure but I think she might now be in the US.
It's hard to know who to suggest without knowing anything about the audience or topic
sorry for the lack of context! In this case, it's a series of talks from external researchers, and the topic has to be something relevant to developmental economics, but no other specific topic otherwise.