I would recommend barley. Genomic resources are available as far as I know and there are lot of data available concerning e.g. pathogens. Nevertheless, if data shall be compared between Arabidopsis and the other plant it would possibly be better to choose B. napus because you stay in the same plant family.
I mentioned B. napus because in my field of research it is really good model as an oil-bearing crops which has fully sequenced genome compared with Olea europaea and so on
Consider Medicago truncatula. NIce seeds, small, sequenced genome, easy to manipulate, decent annotation already. Biochemically, manageable phenolics content, protoplasts stay live long enough to carry on with. Probe (RT-PCR Arrays) and microarrays are already available.
Model plant have to be self-pollinating, with wide range of genetic&phenotypic traits, grown at small pot, fast-cycling, produce large number of seeds. Barley, rice, Medicago truncatula have some genotypes suitable for such purpose. Second dicot should be from solanaceous, brassicas, or Compositae plants. Tomato is very important, but I do not know fast-cycling lines. Brassica rapa has some self-pollinating genotypes (yellow sarson, hiroshimana), complete genome, and it is fast. B. napus has a lot of transposones and alloploid genome, and genetically more narrow comparing to B. rapa.
I guess the original question is almost impossible to answer, because the correct choice will depend on stating first 'a model for what'? Anyone interested in studying nitrogen fixation will choose a legume ( the current international choice being Medicago truncatula), but someone wishing to analyse C4 metabolism will choose another (most likely tropical) model species. The same goes if one is interested in ploidy or gene dosage effects - probably choose a polyploid then. Therefore the right model will depend on the question one is asking, or the traits one is willing to investigate.
My guess is also that the transfer between model plants and standard crop plants is anything but straightforward, even between phylogenetically not so distant species. This is why we're not growing the model plants after all - they're different from the model ones for important characteristics (yeild, crop quality, agronomic performance etc...) that are not easily studied in the model systems.
Finally, I think that the advent of high thoughput genome sequencing technologies will lessen the need for only a few model species, as they make genomic resources available fast and cheap in many organisms now. Relying on model species was very much needed at the start of the genomic era, but will in my opinion become less and less relevant with time.
I agree that the question is rather out of date. I guess that any plant can be a model if it is accepted by many researchers as standard for development of new research methods and experiments in certain field of biology(like nitrogen fixation or plant-pathogen interaction). A good example of an attempt to establish new model plants - rapid cycling brassicas in Wisconsin University (www.fastplants.org/). Despite long-term efforts, there is a limited number of publications about research done on those lines. Probably we should discuss more model genotypes (lines) for each cultivated species, but some crops are hardly suitable for extensive physiologic and genetic experiments due to bi-annual life cycle, high genetic heterozygosity, or large size.