Fiat Panda. Had my 1.2 for three years, averaged 50mpg, and it hasn't missed a beat.
Which one? The mk.II or mk.III?
I bought one mk.II 1.1 Petrol and it has been an amazing car. It was the car that I most wanted to buy, and got one for 5.200€ out of a dealership with just 20.000km!
It's nimble, fun to drive, fits in a lot of narrow streets and can get you further into rough roads than other city cars, even though it's a FWD. Generally maintenance and parts are very cheap to get.
Last week I had an unfortunate accident when crossing a submerged street. Broke the charriot and bent both the transmission and rim. It was fixed in two days and up and running for much less than I anticipated!