How to get best print results from Illustrator?
I'm printing a large poster through a full color inkjet plotter.
What is the best format to export a design from illustrator for print, that will result in the best quality and lowest file size?
PDF format is the most versatile and flexible format. It can be used for both high-end printing (litho) low resolution web use and in between depending on the settings.
In illustrator save dialogue boxes you can set the image compression. I would suggest no higher than 300dpi. You can also set the compression level (generally use "high" but not "very high" jpg compression.
One important tip: use "save as" and make a new file with "preserve illustrator editing capability" switched off otherwise your file will be much larger than necessary. This new file will not be editable but will be optimized for file size and print quality.