I'd say it depends on what you're doing with the file.
PDF is often the easiest, but I've seen some things (shadows, gradients, sometimes even fonts) get lost because the file wasn't prepped correctly. (Yes, that can happen with any file.)
For anything requiring a machine cut (channel or other letters cut via a router, vinyl, etc.), a vector file is a must. Since EPS usually has the best success rate, that's what I ask for. AI will work, too. In my experience, stay away from CMX, because they're choppy as all get out, using CDR requires saving to an older format, and DXF files usually come through either in segments that have to be welded or choppy themselves in the curves.
For printing, raster files are fine- as long as they have sufficient resolution. I usually ask for PSD files for this, because our Flexi (whatever the most recent release is) has a weird habit of crashing on certain TIFF files. PDF can be good for printing, too- but we have Photoshop, so I can convert to a PSD should anything go "wrong" when Flexi imports it.