The 26-year-old endured a disappointing 2013/14 campaign by his own remarkably high standards, as Barca surrendered La Liga and were dumped out of the Champions League in the quarter-finals by Atletico Madrid.
However, Messi still scored 28 times in 31 games, as well as creating a further 11 goals for his team-mates across the course of the season, and Iniesta believes the Argentina international is a one-off.
He said: "Barcelona will never have another player like him ever again and he will never find another club like Barcelona.
"He is a unique talent, everyone knows what he is capable of."
Iniesta could well face Messi at the summer's World Cup, and the midfielder revealed his admiration for another Argentine, in the shape of former Barca star Juan Roman Riquelme.
"To share a changing room with Riquelme was just an absolute pleasure. You learned every day from playing alongside him," he added.
The 30-year-old, who scored the winning goal in the 2010 World Cup final, is set to play a pivotal role for Spain in Brazil, and signalled his intent with a goal in the 2-0 friendly win over Bolivia eight days ago.