The assassination of the Romulan Senate.
I actually had some high hopes after seeing this scene. It felt very in tune with classic Star Trek.
The scene felt dangerous, and held a greater air of mystery than any part of TUC, for example.
I had managed to remain spoiler-free so didn't know anything of what was to come. The exciting prospect of Romulans finally on the big screen and a brand new adventure for the Enterprise
had me hooked.
Then the rest of the movie played (sorry)
Donatra and the Valdore
also had great potential (but were underutilized)