Images and Words changed everything for me, musically, and I am not alone. I do not discount that. But at the end of the day, despite having some of the best songs in their entire discography, it is also inconsistent and has some of their weakest. Awake is similar, but just a hair below. FII has some terrific songs as well, but is ultimately one of their weaker albums. And while I can appreciate WDADU, it is my least favorite.
The Mangini era has three flawless albums, two of which are in my top 4, and a VERY good one that I love dearly in The Astonishing.
Mangini era easily for me. Again, the first 4 is a legendary period of music--especially when you look at where they were about to go next with albums 5 and 6. But going solely off of quality of what I like to listen to, I have to go with the Mangini albums.