best file format for large format printing

can I please get some opinions on formats for large format printing? Initially I used EPS, the downside is that EPS dose not produce a Thumbnail usable in windows, so we moved over to PDF which allows great preview capabilities and small file size but is not 100% reliable and sometimes elements with transparency are not exported, or printed incorrectly, or even a font attached to a path did not get exported.  Now I am using JPG and hope that will work proper however the export time and file size is huge. I do need CMYK and realy would like to keep the vector file for spotcolours used in the printer. Any other experiences or recommendations? or are there plans to get the PDF export 100% working out of corel draw. the same problems exist in version 7

Thanks for your input