Mesaure Up says Differential.
http://www.genie-soft.com/asp/Community/KnowledgeArticle.asp?KBID=105 says Full.
Various technotes and sites say differential is faster than incremental but not which is the fastest out of all three...
If someone could clear this up for me that'd be great.
Btw. I think its Full backup as the other two require the full backup and additional backups to do the restore.