Using only the books as sources, his most flamboyant use of magic occurs off-stage - a good example is his battle with the Ringwraiths at Weathertop. Lots of thunder and lightning, but only seen from afar. His battle with the Balrog on the top of Zirak-Zigil is also very flashy, also with lightning and thunder, but again, it's related by him in flashback. The most outright-powerful use of magic he displays "on stage" occurs as he races out to rescue Faramir and company; he uses a ray of light to drive away the Nazgul almost at the gates of Minas Tirith. But, this could have been an effect of the Elven Ring he wore, which was the Ring of Fire.