Best in what way? The Blood Angels are by far the most noble and most beatific by virtue of Sanguinius. They're also the fighty-est of the more "codex" Space Marine Chapters. The Dark Angels are a bunch of secretive bastards, who are as psychologically scarred as the Blood Angels but are more somber in how they deal with their bloody past. They're by far the shootiest of the Space Marine variants. The Ultramarines are Spartan, bland in every sense of the word, but set a standard with which the entire Adeptus Astartes functions. And for some reason, blue does look quite sharp on the sons of Roboute Guilliman...
As for which Chapter plays the best overall... it's ultimately in the hands of the player. I've seen Ultramarines armies outfight World Eaters in close combat in the hands of a skilled player, and I've seen Blood Angels outshoot Tau. Personally, I think the Black Templars are the best of the variant lists, purely because they combine exceptional close combat skill with what's essentially Codex: Space Marines quality firepower.
