Benji getting to perform at finals with the Treble Makers. He nails it and you can see how overwhelmed and happy he is the whole time he's performing.
It helps that you can literally see his confidence rising as the song goes on. Plus Beca's smile combined with Jesse's tiny encouraging nod, it will melt even the hardest heart.
And then at the very end, he actually realizes what just happened with a wide-eyed stare into the crowd: yes, the crowd is cheering for him, and yes, he did very well. He finally propels himself into a hug with Jesse, who let him accomplish all of it.
Also, considering the at times snarky humor of this movie and of teen movies in general, it would be completely expected for him to totally bomb the song. NOPE. He nails it, and it is awesome.
Beca performing "Don't You Forget About Me" for Jesse to hear.
The last scene, where Beca and her band of sisters is choosing the new wave of Bellas, and she is far more secure in herself and where she is in the world because of what she's experienced over the year. Remember how she was like before the year? She can thank the Bellas for the change.