Re - the Highlander Hybrid - some tests have reported disappointing mpgs.
Your local bookstore has copies of the Consumer Reports New Car Buyers’ Guide. That’ll list all the options currently available as well as good reliability, eneregy efficiency, and cost data. I’d recommend starting your search in this publication, and then test driving all of those that look interesting.
The article that I read that rated the CNG Honda as greenest also commented on why no diesels made the top 12, and the reason why the wide gap from the best diesel to the cars rated “more green”.
In short, even the best diesel falls way short(by comparison) if you are looking to be called “green”.