The two people smarter than Mr. Terrific in the DCAU would be Braniac 5 and Lex Luthor.
In the comics and the DCAU the highest level of intelligence is a 12th Level intellect.
From the Justice League Unlimited season 3 episode # 10 Far From Home:
The Legion of Superheroes has been attacked by the Fatal Five.
BOUNCING BOY: This stinks, Brainy. I hate running from a fight.
BRAINIAC 5: They'll never look for us here. That gives us a chance to counterattack.
BOUNCING BOY: Attack how? There's only the two of us left.
BRAINIAC 5: Yes, I've been giving that problem some thought.
Braniac 5 then uncovers a Time Sphere
BOUNCING BOY: Oh, no. I know you're a level-12 megagenius,
but you are seriously out of your mind.
From Justice League Unlimited Season 3 Episode 13 Destroyer
Darkseid has returned and the Justice League has to team up with the Legion of Doom to stop his invasion of Earth.
In the previous episode Lex Luthor was confronted by Metron of the New Gods who warned him not to go through with his plan to reconstitute Braniac. Luthor ignores the warning and fires up his machine but instead of Braniac it returns Darkseid to existence.
In this episode Luthor confronts Metron and demands that he help them defeat Darkseid. Metron knows of only one thing that Darkseid wants above all else and agrees to take Luthor to it.
METRON: Behold the Source Wall.
Behind it is the single greatest secret of the universe.
This is as far as I dare to go. I warn you one final time.
Only a 12th-level intellect has the slightest hope of surviving what you are about to experience.
LEX LUTHOR: Then I'm overqualified.
Luthor manages to survive his ordeal and returns to confront Darkseid with the thing he desires most The Anti-Life Equation.
So by canon Luthor and Braniac 5 are both 12th level intellects making the the two smartest people in the DCAU. However, if you are trying to limit it to the current earth and just humans then you may not count Braniac 5.
Jim Krieg said in your video about Mr. Terrific
He's not the smartest man in the world. He's the third-smartest man but he's certainly the smartest guy in the Justice League.
That statement would rule out every other hero that has been in the Justice League as being smarter than him. Most of the other characters that people bring up as being the smartest character in the DCU either haven't appeared in the Animated universe or their is no canon source stating how smart they are in the DCAU.