As mentioned, there is no "best" polymer clay for all people and all situations and all objects/techniques. Also, there are various lines of the brands which may have very different characteristics (e.g., "Fimo" and "Sculpey").
They all "bake very well" but if you're thinking about darkening more easily if not protected in various ways, then definitely Kato Polyclay would be best...it's also one of the top brands for doing everything but the push-pull type of "sculpting" though it is firm, which is part of what makes it good.
The main "Sculpeys" are not the best quality polymer clays, but shouldn't "fall apart" unless you mean that parts fall off if not properly attached** or that they can snap off if projecting or thin which is true.
Check out the info I wrote about the various brands and lines of polymer clay in these questions:
https://answersrip.com/question/index?qid=20120505080320AAJyCe8
** http://glassattic.com/polymer/glues-Diluent.htm (click on *Some Bonding Techniques*)
.