It's a very real debate, especially when you argue on a "Healthy" Itachi. It's not a hidden bit of subtext that Zetsu repeatedly gassed up Itachi beyond Pein's level of hype despite seeing Pein fight in the same arc.
Itachi's faster than Pein, he typically has better jutsu, he's more intelligent as well.
I'd argue that a Healthy Itachi beats Pein 7/10 times. A Sick Itachi gets close to 50/50.