Rexflame

Rexflame was the strongest Fire magic tome in Fire Emblem:Radiant Dawn. It was the SS-rank for Fire magic, as well as having only 15 uses, and therefore is definetly the strongest tome.

The people that could use it are Tormod(who you get halfway through Part 1, though it seems impossible for him to get Rexflame as you get him back very late in the game unless you use a lot of experience to get him to lvl 10 and then use a Master Crown on him, followed by an Arms Scroll), Calill (Tormod's teacher, who you get on the last chapter of Part 2), and Sanaki (The Empress of the Begnion Empire and was able to wield the Preferential ultimate fire tome Cymbeline, which was slightly weaker than Rexflame by about 1 point of power).