Well, to me, you really have to look at it both ways. You could say it's all about the MC, but if the beat dont fit the MC or the MC is tight, but he's not feeling the beat, it wont sound quite right.
I think both have to be well coordinated and then, the beat OR the lyrics can be simple and the beat will pop!
The best example to me is that song "My Neck, by back..." I haaaaate that song, but it did have his 15 minutes of glory... And as Complexx said, Dre is the man when it comes to simple beats...
So it can really go both ways
of course, simple beat - phat lyrics will always win over simple beat-simple lyrics, but what's better?
Phat Beat - Simple lyrics or
Simple beat - Phat lyrics?