Personally, I don't think that Lucas's "Anakin" character as portrayed by both Jake Lloyd and Hayden Christenson, really manages to convey a convincing picture of someone capable of "becoming" Darth Vader.
So the question is: which character, fiction or non-fiction, has what it takes to be a more likely candidate to becoming Vader?
After watching A League of Their Own today, I realized that the character of Kit Keller (Lori Petty) has more reason to "become" Darth Vader than Anakin Skywalker does. While a lot of her ranting and raving is just as unjustified as Anakin's is (Hayden), her internal conflict was far more convincing in one film than Anakin's will wind up being over 3.
My second nomination is, oddly enough, Sam Monroe from Life As A House, which just so happened to be played by Hayden Christenson. Again, in the course of one film, this character managed to convey a sense of internal turmoil over events which he has allowed to rule his life. The choices he makes as a result tend to be self-destructive in nature. The film shows the character being redeemed in the end, but he very easily could have continued down a "dark path." I think that Sam Monroe would have made a better Darth Vader than Anakin will.
My third nomination is from the non-fiction world. After reading a rather sad account of his life today in the newspaper, I believe that Saddam Hussein essentially became a real life Darth Vader. Not for any forced and concocted reasons, as we saw in AOTC, but for a very real and emotional molding that took place early on in his life. I'd wager a bet to say that most demagogues followed a similar pattern of abandonment, abuse, and fanatical upbringing which takes them to a point where they are unable or unwilling to fully empathize with their "victims." I think that we clearly see that sort of quality in Darth Vader throughout the OT, yet the Prequel version of Anakin does not adequately set up a childhood which would lead to such a monster.
Detective Alonzo Harris (Denzel Washington) in Training Day is a man without true empathy. Though we don't know much about him prior to meeting him when the movie starts, it is clear that he lacks the capacity to comprehend the true ramifications of his selfish acts. Similar to the bratty behavior of Anakin in AOTC, yet somehow far more believable. Whether that is because we simply do not know why he is the way he is, or because of the stellar performance by Denzel, I believe that Alonzo Harris would have made a far more believable Darth Vader.