OK, this is an interesting topic that I hadn't much thought about. I think it could make for some interesting discussion, and frankly I am not at all sure about who is the Browns' best draft pick ever. But here's why I think Thomas may arguably turn out to be the best pick the Browns ever made.
First, I know a lot of people are going to scream at the top of their lungs: JIM BROWN. Yes, yes. Jim Brown is in my opinion (although I never saw him play) and the opinion of a clear plurality (if not majority) of "experts" the greatest NFL football player ever. However, recall that the Browns actually didn't want to draft him; they got him by accident and really wanted some quarterback (though I can't find a cite for that after googling it). So by my reckoning, Brown was certainly the best player ever drafted by the Browns, he was not the best pick by the Browns, because he was not even our first choice.
Second, I know a lot of people are going to say Kosar. This one is a tougher call. Bernie was certainly a great pick in the supplemental draft, and the Browns had to pull out all the stops to fenagle their way into getting him. However, getting Bernie was a no-brainer (assuming we were able to manipulate the draft/supplemental draft system to get him). He wanted to play in Cleveland, and he helped us manipulate the system into getting him.
So, here's why I say Thomas may turn out to be the best pick the Browns ever made. A solid, top-notch O-lineman can be invaluable. No doubt about it; he can step up the play of both our QB's and our running game up a notch just by having him. However, O-linemen are not "sexy" players, and they are not usually taken so high (though it happens of course). Furthermore, good O-lineman last a long time (one of the best ever, Jackie Slater, played until he was 40 for the Rams). The fans were clearly split on taking him, with a lot of people actually thinking we should take Quinn at that pick (which would've been crazy, in my opinion -- see, e.g., Tim Couch - and a QB without an O-line is not very valuable). The Browns clearly sacrificed the marketing of an exciting offensive or defensive player in favor of what could turn out to be a HOF O-lineman. It was a genius choice, but it was not a no-brainer at all.
What do other people think? I know there are probably a bunch of other people I am not thinking about, but I think there is a chance that picking Joe Thomas will be seen as one of the best moves the Browns ever made. . . .