Unlimited Users Blogging System Recommendation?

I am looking for a blogging system. Obviously it needs to be open source so i can modify it to my own needs. I have worked with Wordpress MU and other systems similar, but i am still unsure which is the best. Basically i just need a system where users have a blog and a profile. Structure and good foundation is what i am after here, as i've noticed so many that are just bare scripts and dont really take a emphasis on security and structure. So any suggestions would be nice.