There is no perfect one, use several of them.
- http://mvnrepository.com/
Some Nexus repositories:
- http://repository.apache.org/
- https://repository.jboss.org/nexus/
- http://repository.sonatype.org/
- https://oss.sonatype.org/
Down or no longer available:
- http://www.jarvana.com/
- http://www.artifact-repository.org/
- http://maven.ozacc.com/
- http://www.mavenreposearch.com/
I use most of time http://www.jarvana.com/ or http://mavensearch.net/ and the above Nexus repositories.
