The biggest it could go for would probably be Diplodocus, which isn't that big or tall, and a Tyrannosaurus could probably end the fight by simply dealing a heavy blow to the neck or back.

If you remember Battles that Probably Happened, their tails have incredible whomping capabilities.

It definitely kill something like a Nigersaurus or Europasaurus, but would lose to someting like a dreanoughtus or Alamosaurus.

Well it's not like Diplodocus are fidget spinners that can instantly turn around to whomp a rex.

Tyrannosaurus would be helpless against a Diplodocus since it can't reach the neck.

Like I said it's not that tall.

Both Diplo and Rex(Or Imperator) were 4 meters tall

I thought Diplodocus was 6 meters at the shoulder. I should probably look into this more.

It probably would have to be one of the smaller ones or ones with their necks lower to the ground so it could bite it.

Possibility of taking down one such as alamosaurus, if there were multiple Tyrannosaurus. Although they weren't built for it

If one T-Rex can do it alone, it'll fail

And if many do it together, they'll succeed