which pundits said it: never heard it from gary neviile or keano or rio or even souness or carragher
Several of those exact pundits, for a start. You can hear Keane and Rio say he's the best goalkeeper in the world in the below video (along with the likes of Lampard, Fletcher, Given, Scholes, etc)
And Carragher regularly did. Like here:
“From what I see, if I was picking a world XI then David de Gea would be in it. We are lucky that we have the Premier League’s greatest-ever goalkeeper here,” he added, sitting alongside Schmeichel, “but I think De Gea is second."
Here:
Unfortunately I am going to have to start with a Manchester United player [in my team of the season], who I actually think is the best keeper in the world at the moment.”
Here (while talking about Alison):
Klopp is reaping the rewards of signing a goalkeeper with the potential to become the world's best. When was the last time an Anfield keeper was staking such a claim? For the last three years this title has been David De Gea's. The performance he gave away at Arsenal last season was the best I have ever seen from a goalkeeper.
And so on. If you watch back videos of De Gea's highlights from that time with commentary you'll also hear commentators describe him as "the best in the world", "arguably the best in the world", etc.
As I say, you can think they were all wrong to think that. But it's weird that you missed it being said given you presumably followed us throughout that period where he was the PL goalkeeper of the year in five out of six years, which naturally generated that discussion.