Depends on market and where you do your most driving.
I have a TDI and love it for hypermiling but my driving profile (short, local journeys mostly with occasional longer trips) would dictate something with a higher urban MPG rating and not as much a motorway cruiser.
That said a 1.3-1.4 Diesel in a tiny hatchback body would be superb.
The new Ford KA seems to fit that bill nicely, its a rebodied FIAT 500 but cheaper and available with the FIAT 1.3 Multijet for some 70+ MPG goodness...
