"Enigmatic" might be the best -- and only -- single word that can describe Noah Bennett. During the first half of the first season, Bennett was assumed to be one of the villains of the series, an agent of a highly organized and effective covert operation designed to hunt down and capture "evolved humans". Since then, we have discovered that he has allegiances even stronger than his loyalty to the mysterious 'company', and that is to his family.
Bennett is my absolute favorite character on Heroes (bumping Captain Badass himself, Sylar, to second place ). Why? Because he is an extremely unpredictable character, much like Sylar, and in stark contrast to the willfully naive and gullible Peter Petrelli. Bennett is also frightfully calculating and extaordinarily clever (another contrast to Peter). His battle of wills and his constant attempts to maintain the equilibrium between his love for his family and the demands of his employer make for great drama.
Peter is very cool; it's nice to see a television show depict the "nice guy" as more than just a well-intending schmuck. He's potentially the most powerful of all the evolved humans. The only detraction is, as I said, he's not among the greatest thinkers or tactitions. Sylar and Bennett outclass him in the mental aspects, as does his (presumed) late brother.