No 'uhh...' about it, the Sjengster undoubtedly has the finest single-hander in the game today. Guga and Haas' backhands may well be hit harder, but they're less compact and flat than Schalken's and they're not as consistent as his one-hander.
For double-hander, it's a toss-up between Agassi and Safin, but again it's a hard choice - for sheer pace I'd go for Safin, for crispness of shot I'd go for Agassi. Yeah, AA just has it.