There are many songs that could claim this title. In fact you can get an album of Britpop anthems which includes everything from Paul Weller to Blur - it has sold well, but a big surprise to many music lovers is that it contains nothing by Oasis, who are the first name many of us would think of in connection with Britpop. In fact if you are looking for a Britpop anthem, Oasis' Don't Look Back in Anger or Wonderwall would get a lot of votes.
The other biggest names associated with the genre are Blur and Pulp. Pulp's Common People would be my own choice, and different numbers by Pulp feature in most Top 10 lists, along with Song 2, She's So High or Beetlebum from Blur.
But these are just the best known choices - to get an idea of the variety of opinions, you could put 'top 5 britpop anthems' in a search engine and explore some of the discussions on this topic.