So when we try to answer which is the best of their era, there's potential for a huge shift in preference depending upon the years specified. Personally, I see Byrne's influence as the 'top artist' of the era ending shortly after FF 250. The art looked more rushed, lacked much of the finer details (to my eyes), and became just 'OK'. Suddenly, I didn't feel compelled to pick up the latest Byrne FF book or subsequent titles, and I started to stay away from his stuff with Namor, She-Hulk, etc. There were other artists who proved far more interesting to me at that time, and anything they worked on got my money.
As Byrne started his descent, I felt it was right around the time I truly noticed how great Perez was, not just on NTT, but Crisis, and a host of other projects he dabbled in.