i would have to say that any lake in texas can be argued in this. Its all in finding the fish. My favorite lakes are so small that almost no one has heard of them, yet they produce alot of catfish. Hords Creek for channel cats, OH Ivie for yellow cat, and Lake waco for blue (not so sure about lake waco but only place i have really caught alot in one day.)
This could just be a regional popularity contest.... I've been practicing for retirement for the past two years and getting ready to be a full time catfisherman. I say Conroe for channels (lots of little ones), Houston for blues and toss up between Texana and Coleto Creek for yellows. Although you really have to hide your lines on Coleto from the bass fishermen - they're fond of cutting them and sinking them when they snag them with their lures. Guess that settles it for me - best flathead lake is Texana.