Anyone wondering about Pepin's abilities as a line chef ought to read his memoir, The Apprentice, which is wonderfullly written and delightfully uninterested in self-promotion. Quite apart from working as personal chef to two French presidents, including De Gaulle, he did long stints at the Palace Athenee and a number of other really high-end places, in France and in New York. He gave it up because (a) he found teaching a lot of fun, and (b) he was in a horrendous car crash which left him unable to do the really heavy labor as often as it needs to get done in a high-volume kitchen. By the time the new culinary revolution hit -- something in which he and Julia Child were prime contributors and motivators -- he was hardly young, and had been trained in the very haute cuisine that nobody now wanted. He now teaches at the French Culinary Institute in New York and at Boston University, does TV shows, and generally works amazingly hard for a guy who got drafted in the Algerian war: I don't have the book with me, but based on when that war was, he must be in his 70s.
This is why I think the "best chef" thing sort of mixes apples and oranges. Batali and so on are young hotshots, and Pepin is at least one generation back. Who's the best? When, at their prime, or now? I mean, would we agree by that measure that Careme is a truly awful chef? -- he's been dead for more than a century, after all, and isn't keeping up with the times.
I think the only way to compare reasonably is to compare people in their prime right now, in which case I'd go with Batali and Tsai, assuming we don't count the little passing bits with Ripert as "TV celebrity chef"-dom. If we do count that, Ripert is the best, and has the stars to prove it.
If we're looking at the world, of course, the best TV celebrity chef is Murata Yoshihiro, who appears regularly here on Japanese TV and is one of the top three kaiseki chefs. If they ever do an Iron Chef Japan-America, and get a bunch of judges who actually know what they're talking about with both Japanese and American food -- not just some sushi idiot but really knowing Japanese food -- I'd love to see Tanigawa or Murata on it. God help the poor "Iron Chef" who'll be crushed like a bug by these guys. Tanigawa slaughtered Morimoto on the original show, and Murata has a much better range for international cuisine.
So there are my votes, depending on how we're counting:
Pepin, if it has to be regular TV shows, not occasional appearances, and we consider one's prime rather than current ability.
Batali, if it has to be current ability and regular TV show.
Ripert, if it's current ability and semi-regular American TV counts.
Murata, if it's current ability and semi-regular TV anywhere counts.