The one the government did not support, plan for, or subsidize.
It increasingly looks like hybrids, particularly newer plug-in hybrids, will be the high MPG, low-emission technology winner in the foreseeable future. The US and California governments, among others, have subsidized (and at times mandated) pure electric vehicles, hydrogen vehicles, natural gas powered vehicles, and fuel cell powered vehicles. While some governments have come along with ex-post-facto promotions of hybrids (e.g. ability to use the carpool lane), hybrids have been developed and won in the market entirely without government help and in places like California, effectively in the face of government opposition (because they were stuck on zero emission vehicles, low-emission vehicles were opposed)
Plug in hybrids have many of the advantages of electric vehicles without the range problems. They use standard gasoline so they avoid the new fuel distribution issues of natural gas and hydrogen. And fuel cell technology may be great one day but is not there yet. I was reminded of all this by this article by Stephen Bainbridge on why the EV-1 failed.
Update: This reminds me of the 19th century transcontinental railroads - UP, SP, NP, GN etc. Only one of these transcontinentals did not get any federal land grants or government financing -- the Great Northern of James J. Hill -- and not coincidently the GN was the only one not to go bankrupt in the close of the century.