Since 1987, U2 has to be considered the biggest band in the WORLD, since no other group has matched their popularity around the globe --- solo artists have, but not a whole band. Oasis might've been big in the UK, but they never really caught on in North America (or at least the USA) in a big way, and vice versa for the Dave Matthews Band.
If you look at it in terms of consistent popularity and quality, nobody has pushed U2 aside as 'the' band, since nobody seems to be able to release more than 2 quality albums back-to-back anymore.