I have used plot files (HPGL aka *.plt) and .pdf (Adobe Acrobat format) successfully. Each has pros and cons but .pdf is probably the best bet for the long run. HP is no longer including HPGL as a standard feature on their large format printers (which they no longer call plotters) which suggests to me that it is on the way out.
PDF files are larger and you have to be careful about their tendency to be reduced to fit the printer's margins, but they have the advantage of always printing as remotely as they print locally and as they appear on screen.
PLT files are smaller (sometimes MUCH smaller) but they are dependent on the remote service setting up their drivers correctly. This SHOULD be very simple (it's just a matter of letting the file determine all the settings) but is nevertheless often a problem. One time it was impossible to to talk them through it (there was also a bit of a language problem) but the PDFs worked fine.