Security breach & spam ads, best security plugins / solutions?

One of the sites I volunteer as a webmaster is having some recurring security issues. It appears somebody is getting access to the site and placing ads under the header. Seems pretty silly especially since half the time they are dead links. Anyways, I've run updates, re-issued more secure admin passwords and a few other basic steps, the problem subsided but then returned.

Do we have any wpmudev security plugins to increase security, or if not, what are everyone's current favorites?

Thanks