South American Red-foot tortoises are great! Perosnality, IQ, strength, colorfulness...
On the other hand, the Carolinian Box turtle is gorgeous- fascinating patterns, interesting habits (worm stomping!)...
Wood turtles are cool, too, as are Fly River turtles and Ridley's turtle...
Mata-matas withtheir incredible hunting technique, and Alligator Snappers and their 'bait tongues'... and side-neck turtles, and the odd Hinge-Back tortoises, and the soft-shelled Pancake tortoise...
Giant tortoises and the big sea turtles are neat, and the various 'star' tortoises are beautiful! Softshells are interesting, and some of them have incredible shell coloration!
I guess it sort of depends on what you mean by 'best'?