While guys like Chuck and Tito held the UFC together and brought it some base attention up to 2005, it wasn't until GSP came along that we saw the UFC become the massive name it is. When you look at the peak year's of the UFC, it's mostly tied to when GSP was fighting.
I will give all credit to TUF and Spike for bringing the UFC to the forefront, but when GSP beat Hughes in March of 2006, his persona and style really put the UFC over the top. Ratings blew up and even moderate MMA fans began paying attention to other names. Brock's addition to the UFC only helped thing, but ratings didn't drop after he retired. But it wasn't until GSP's last fight in late 2013 ratings began taking a hit.