Best Plug-Ins

Hi All,
I've just joined this site. I noticed that in addition to the great plugin on WPMU Premium, the free-blue WPMU Dev has many open source plugins.

Here are my questions:

What do you see as the best/most useful WPMU Dev Premium Plugins?

What do you see as the best/most useful WPMU Free Plugins?

What plugins on WPMU Free do you see as being especially buggy, security risks or just not that useful?