You wouldn’t say The Mets is my favourite team, would you? It’d be wrong, and sound strange, wouldn’t it?
But what about this:
Manchester United is my favourite team, or Manchester United are my favourite team?
I imagine that some of you chose the first, and some the second. And as is often the case with English, both are correct. Though how natural it sounds depends on where you’re from. Generally in British English, groups are referred to as plural words, but in American English they’re treated as singular words (unless the words are clearly plural with an s at the end, like the Mets or The Beatles).
One can see this not only with sports teams, but also with bands (Radiohead is/are my favourite band), and companies (McDonald’s is/are a big business), though it’s also possible for people to refer to companies as singular in British English.
It’s just another of those small, probably-not-very-significant differences between American and British English. It always sounds a little strange to me when I hear the American way, but it’s nothing I really get excited about. I wonder though, if this little difference says something about some of the greater cultural differences between the United States and Europe. The US has was founded upon the ideals of personal liberty, and that streak of individualism, that anyone can make it if they’re given the chance, has always been a part of American culture.
In Europe though, more communitarian or socialist politics have always had a more mainstream place in society. Maybe the different ways we refer to groups reflects this. Or maybe not, it can be too easy sometimes to read too much into things, and perhaps the different forms just sound better in different accents.