Currently, I've fallen in love with this relatively newish rice-and-noodle bar called Yam Yam. I think they're only available in London for the time being, but they're really good and allow you to pick and choose curries, sauces, rice, noodles, anything pan-Asiany like that. However, there're only about 3-4 of them in the entire country, which seems unusual considering how good they are.
After that, it's probably Quiznos, but there are also very few of them as well, so most of the time I have to settle for Subway (which is fine, but not the same).
Out of the options mentioned, it'd have to be McDonalds, due to their wider choice and lower prices, at least in the UK. Their fries are also considerably nicer as well.