Cjedi, Hayden's "whining" and general poutiness in certain parts of AOTC and ROTS is less to do with his actual acting and more to do with the way the director chose to conceive the character. Some people think Dicaprio would have been a better choice, and while i do indeed find him to be a better actor, it would not have made the difference--instead you would have Dicaprio, another fine actor, rendered into an annoying brat whining "its all Obi Wan's fault! *pout*". Thats what it comes down to. As you said, if Lucas made the character older--more mature--he could have had better and more compelling options; but he didn't--he chose to characterise him as young and immature, and no mater who filled the shoes of Anakin Skywalker he would have still had to portray a young and immaturely characterised person. The conception and dialog that Lucas chose to shape Anakin with turned him into a whiney poutey sissy, and Christenson did what he could--and he even admits that he was utterly perplexed why Lucas was pushing him in that direction, instead wanting to play it cooler, more controlled and Vader-ish. As for Christenson's other roles--yes, he is typecast as the "angsty/troubled teen" and so the whininess is often seen. But thats not a limit of his acting ability--thats an inherant flaw of the characters he is unfortunately stuck playing. In the scenes where he is granted more realistic characterisation where he can show his range, he is utterly convincing--perhaps not brilliant, but certainly above-average (whatever the case, not flat-out bad).

Portman too, while not always someone who will blow you away with a performance, usually delivers ones that are natural and that make you forget you are watching an actor pretend or read from a script (unless you are constantly directed by the self-awareness of her celebrity, which is a common roadblock). Films like Leon, Closer, Beautiful Girls, Cold Mountain and Garden State all show what range and believabiliy she can project, even if not all of these examples are necessarily in the same category as the likes of Neeson or August.
She was i think 16 at the time she made TPM, and 18 or so by the time AOTC was shot, and of the available actresses to play a role of that age bracket, i can't think of anyone greater than her.

Samuel L Jackson too is a great performer--he has a larger-than-life personality that is often self-aware of his own image, and this frequently needs to be controlled and shaped--and used to proper benefit-- by a competant director in order to render a good character out of him. Films like Pulp Fiction, Jungle Fever and A Time to Kill show him at his best, but ever lesser efforts like Changing Lanes, Sphere or Jurrassic Park show the variety and subtle characterisation changes that can be wrought out of him.