I'm trying to come up with a phrase that could replace "frontest" in "Look at that man in the frontest row," as frontest is not officially correct grammar. I know I can say "Look at that man in the farthest back row," but I can't figure out a decent way to imply the opposite.
"Farthest in the front" is the opposite, and very much an option.
That said, I don't think anybody actually says "in the farthest back row". You'd say "in the last row" instead, or "in the very last row" if you need additional emphasis.
Likewise, the frontmost row is just called the first row. Or the very first row. Or, well, the frontmost row if you absolutely have to.