It beat World of WarCraft and StarCraft II.
SC2: Beta began 17.2.2010, and ran until about a week to its release, which was 27.7.2010
WoW: Beta began 19.3.2004, and ran right up until release on 23.11.2004
D3: Beta began (publicly) 20.9.2011 and will run until 1.5.2012
It has also beaten every other Blizzard game from days of announcement to opening of the beta by about 170 days, and it will also be Blizzard's longest public development.