What was the tallest dinosaur?

It's difficult to say at this point. A lot of the largest and possibly tallest dinosaurs aren't very complete.

BrachiOSaurus is still the most complete large dinosaur which we can say with reasonable certainly was tall. It might have been able to reach about 14m in height if it's posed with an elevated neck.

There is another brachiOSaur called Sauroposeidon which is only known from 4 neck bones. These are very long (the largest bone is 1.4m long). It suggests a dinosaur with a 11 to 12m neck. There are a lot of uncertainties as to what the rest of it looked like so it's default to know exactly how tall it was. Assuming its body was built like that of BrachiOSaurus, and it held its neck in an elevated pose, it might have been able to reach about 17m in height.
Sauropoisedon
suropods.