Jeremy stage-diving at the Opera during High Fidelity.
Heck, way before that - Jeremy breaking into Team Templars' van and screwing pretty much everything up that they had planned.
We couldn't have this page without a mention of Upstairs, now could we?
A large part of the opera scene in High Fidelity, for one.
The climax of With A Bullet. Simon and Farraday are struggling. Simon's lost his gun, but Farraday still has his. Farraday pulls his gun on Simon, who hears the gun go off, but is surprised to see Farraday fall to the ground with his brains splattered all around. Turns out Jeremy, who'd been kidnapped, sneaked over and picked up Simon's gun, then shot the bastard with perfect form, all after one shooting lesson. Although, to be fair, he had no idea how to hand the gun over afterwards, nor hold it beforehand.
And Jeremy also mentions much earlier, when Simon remarks on his excellent aim despite never having touched a gun before, that he does have quite a bit of experience with properly gauging distance and trajectory with other things, such as his gas shooter.
Jeremy's entire scene in Chapter 24 of High Fidelity once the team bursts into the hostel room. In less than ten minutes, not only does he refuse to back down in the face of Sandra's fury, but matches it, he flatly tells everyone to essentially shut the fuck up, gets Simon to back away with just a glare, and handles the phone conversation with the Russian in a way that's equal parts awesome and terrifying.
Jeremy: You— [taps Johnny's shoulder] —keep an eye out the window, let me know if anyone enters the alley, you— [points at Simon] —listen at the door, and the rest of you had damned well better not exist.
And they listen, possibly because in that moment Jeremy was pretty much a thousand times scarier than Sandra or Simon ever was.