hmmm. I haven't even messed with eBooks for the iPhone yet, so I don't know either, but that's part of the issue of generating formats only when I need them. So I'm not locked into any one device or eBook vendor and the availability of any particular title is maximized and available for conversion to any format for any device.
I guess a more exact question is:
Within calibre at present, what input format would allow the best conversions to ANY other format? I'd guess on ePub, but I'm not sure. I'm new at this but so far ePub seems the cleanest.