It is a big derby, not sure about the biggest derby in England. Derbies are awesome but they are all unique. Newcastle Sunderland has a lot of history behind it dating back hundreds of years ago but in footballing terms not many people around the world care about it, merseyside derby is similar to the manchester derby as it contains two teams that play in blue and red, both red teams hate each other as well (in what is probably a bigger match world wide) but I would say is a much friendlier event than the manchester derby. West ham millwall is a violent, nasty affair so too is the Birmingham derby. There are plenty more as well filled with banter, rivalry and pure hatred.
The most important derby in footballing terms is the manchester derby at the moment.