If someone else was paying the running costs, I'd have the biggest diesel engine I could get, then tune the hell out of it.
Nothing's going to change the fact that all my driving is motorway and dual carriageway, with a few miles across town here and there to get from home to the main roads. I really like the low down torque of a big diesel engine, and I haven't found a petrol yet that gives the same feel. If I had a 20 mile drive along empty, country A and B roads each day, it would be a different answer.
Just how many Yorkshire sheep can you fit inside one exhaust?