BuddyPress.org

@Anointed: It’s not that the core is “flawed”… it’s that BuddyPress is simply a plugin that sits “on top” of WordPress MU while Elgg is an integrated solution. Personally, I still prefer BuddyPress. The theming process in Elgg caused me great pain and I find the Elgg interface suffers from inconsistencies and poor usability.