I'm a physician scientist and generate several posters a year. I'd suggest Pages over Keynote over Powerpoint. But I'd suggest InDesign over all of these. Pricey yes, but wonderful control over every aspect of the printed page.
Whichever one you choose, make sure you save/export your poster as a PDF file. Almost all print services understand PDF files, and you can preview your file (using Preview, of course) to make sure it's right before submission. PDF files preserve your fonts, etc., and that's what you want.
For scientific graphics (e.g., error bars, etc) a previous commenter is right: you need a scientific graphics program. Pick your favorite and use it. You can import the graphic as a PDF, TIFF, or EPS file, depending on the program you use. My needs are straightforward, and Deltagraph works well for me.
A final suggestion: don't start your poster the night before it's due 🙂