I admittedly haven't had enough experience with 6, but for now I can say with fair confidence that Bloons TD5 is the crown jewel of the whole series. Utterly blowing the tacky and clunky 4 out of the water entirely, Returning to the more cartoonish aesthetics of 3, this game seems to have the perfect roster of towers and infinite replay value through its bountiful challenges and insane breadth of difficulty options.