I love both. Yeah, The Simpsons isn't as good of a show as it used to be, but when you've been doing a show for 18 years, it can be hard to maintain that standard.
I love Family Guy, but I have to vote for the Simpsons from the sheer fact that there most likely would be no Family Guy without them. Family Guy immitates The Simpsons' way of storytelling a lot. Not that other shows don't either, and not that it's a bad thing.