In terms of caliber and ballistic energy, .50 Action Express (the largest caliber size that the Desert Eagle is available in) is the largest commercially available handgun cartridge on the market today, It is mostly used by big game hunters and gun enthusiasts, special forces operators tend to shun the Deagle for it's heavy recoil, small magazine size, and poor tactical flexibility (it's heavy thus harder to draw and aim). Most operators find that the .45 and 9mm calibers are sufficient for most operations.
In answering the question, yes, it is the most powerful pistol in the world, except perhaps for certain "assault pistols" which are just assault rifles with stocks removed.
The Desert Eagle is a pistol powerhouse in its .50 AE caliber,but in terms of most powerful and accurate,I'd say that it's the second best. The handgun that beats it is the Smith & Wesson Model 500 revolver,which fires the .500 magnum round. .500 mag = 3 .44 mag rounds power and accuracy. .50 AE = 2 .357 magnum rounds power and accuracy