It's very unfortunate that you have to make one definitive choice over any other, and I can't really pick out one piece that is the "best" of the line. Reason being that they are all sterling in terms of depicting the animals as we currently believe them to have either appeared or to plausibly behaved. You could literally spend days critiquing each individual piece and elucidating the thought that went into every single minutia of detail, be it the posture of the dinosaur, the color scheme, the folds in the skin, scar tissue, dentition, or even simply the expression and/or gaze of the creature(s). Hell, even the bases seem to convey a great deal about what is going on in the statue.
The diorama is pure classicism in terms of its depiction, and its dynamism, unfettered grace, and scathing starkness of the violence of such conflicts makes it a definite favorite. That, and it's the only piece out thus far, and we know that it delivers in every single department, be it sculpt, paint apps, etc. The Carnotaurus is a bold choice and is definitely meritorious for its originality, as is the Styracosaurus. The Tyrannosaurus is my personal favorite, reasons of nostalgia aside; just a cursory glance at the creature is as informative as a storybook, relaying epic and near fatal moments of its time in this world; that, and it is a true alpha predator - an unchallenged, dominant, aged combatant with literally no rival in its domain. It evinces that, much like Estuarine crocodiles, age in dinosaurs should not necessarily convey dotage as it does in humans. Rather it can merely heighten experience, wisdom, and skill of a combatant on the prehistoric field of battle. It's that concept that I find appealing, so my vote goes for the Tyrannosaur, but I really again would prefer not to choose.