Superman told the truth - Mr. Terrific has the Dark Knight beat when it comes to intellectual prowess.
DC Comics is filled with genius-level characters, whether it's Batman, Mr. Terrific, Lex Luthor, Ray Palmer, or Ted Kord. The list is extensive. But, when it comes to the mighty Justice League, Michael Holt aka miniseries to learn more about the character. So, what does Superman think? He's team Terrific all the way.
In Superman Up In The Sky #6 by Tom King, Andy Kubert, Sandra Hope, Brad Anderson, and Clayton Cowles, Superman successfully tracks down a young girl who was captured from Metropolis into the deepest corners of space. After rescuing her from aliens, Kal-El travels with her across the cosmos to return to Earth. The young girl is very inquisitive, asking Superman as many questions as he can handle. She asks him how he can fly, who his favorite Robin is (the original, Dick Grayson), if he made Lex Luthor bald ("that's what he thinks"), and lastly, if Mr. Terrific is smarter than Batman.
The young girl asks, "is Mr. Terrific smarter than Batman?" Superman responds, by saying "he says he's not," which the youngster asks "and you believe him?" Kal-El tells her that he does not. The shows Superman knows how smart Mr. Terrific truly is, as he's not slighting Batman with the response - Holt is just that smart. Batman can outsmart most heroes and villains, but when it comes to pure competition of intellect and genius, Mr. Terrific wins - and it's not particularly close.
Now, would Superman tell Batman to his face that he's not as smart as Mr. Terrific? That's doubtful. It's safe to assume he wouldn't want to hurt his friend's ego. Mr. Terrific might say he's not smarter than Batman - and it's possible Bruce Wayne would it Terrific has him edged out - but that's just the hero being kind towards his fellow Justice Leaguer as there are few smarter heroes than him - even the Caped Crusader.
