Hmm? By rexes do you mean other large theropods?(Because Rex is definitely the biggest Tyrannosaur.) Giganotosaurus is slightly longer and taller.(Depending on the indivual. Same goes for its very close relative Carcharodontosaurus) But it's not nearly as robust as T. Rex.
Just taking a look of the heads of both shows how different they are. Top being Carcharodontosaurus the bottom being Tyrannosaurus.
It may look like Carcharodontosaurus is more imposing due to the longer snout and more teeth. But really T-Rex was a much more advanced and powerful predator. Unlike Carcharo who could only see you with one eye and had a huge blindspot(it likely had to rely mostly on other senses, but T. Rex also had amazing senses other than eyesight), Tyrannosaurus likely had the greatest vision of any animal. The clarity of its vision was 13x times greater than a human's, an eagle's is 8 for comparison. And here's a comparison of a T. Rex's field of view compared to ours.
Then on top of that, unlike most theropods like Carchara which had razor sharp teeth meant for cutting through flesh, T Rex's jaws were built for crushing, it had a bite force of around 8,000 pounds.
TBH, Indominus is visually interchangeable with most other large theropods aside from it having much longer arms and special abilities.
But the funny thing about T. Rex is that the actual animal is a lot deadlier than the more monster-like movie one.