My favourite is the Jurassic Park roar.
It doesn't sound like the other roars that were used for T-rex in previous movies. It sounds a lot more powerful and terrifying, making the others pale in comparison, and because of that, it gained its own iconic status, similar to how original and unique Godzilla's roar is. It has become so iconic that those sound effects are used in a lot of media, like other dinosaur cartoons, Toy Story 3, and Avatar (coupled with some Spinosaurus roars from JP3).
Along with its uniqueness, it has that something special where if you listen closely, you can feel the roar penetrating into your chest. I don't know if that's only me with my poetic justice, but it sounds a lot more powerful than other T-rex roars I've heard.
And, in the JP novel, Crichton did write that the T-rex's roar was "a scream from some other world." I think what he meant was that no one had ever heard a roar like it ever before. Spielberg picked up on that cue, and gave us a roar for the ages.