He also said Manchester United made a big mistake by missing out on a £5m deal for the England midfielder.
Redknapp, who is working a pundit with BT Sport, also insisted that his former team are favourites to win the Europa League, if coach Mauricio Pochettino picks his strongest side.
Spurs travel to Gent in Belgium on Thursday in the last 32 of the competition.
“I think he’s fantastic,” Redknapp said of Alli.
“Can you tell me how all those scouts failed to spot him when he was playing for MK Dons every week? How did they end up waiting until Tottenham came in for him?
“It’s incredible that no-one took him. What a player, what a signing — what a steal at £5m.
“I think he’s worth £100m now – if Pogba’s worth £90m, Alli is worth more.
“He’s just 20 years of age, for heaven’s sake, and the kid’s got everything. He scores goals, makes goals, he can dribble, head it, he’s got an edge to him and puts his foot in… I think he’s fantastic.
“Didn’t he play for MK Dons against Manchester United when they beat them 4-0 in the League Cup? United should have signed him there and then. At least they would have gone home disappointed with the result but they would have had a future star for the club with them on the bus.”