If Tobirama had been more fleshed out, he'd be no.1 in my opinion because he's got a very complex story with complex decisions that have affected the story in more facets than we'd like to think. Kakashi comes very close due to his story having different shades allowing one to peek not just one facet of Kakashi but all the character-identifying traits that have made Kakashi to be an overall likeable character. Sasuke has some let downs especially when they experiment with his emotions a lot and give him all sorts of fury. However when he recovers and is back to his old self he regains his symbol and status he acquired so flawlessly in Part 1. Minato also registers as a top 5 as a debut hero whose inaugural feats set the power ceiling and brought hype all-round to the first half of the series (without him bringing in all that hype, I can't see much to look forward to before characters like Itachi and Jiraiya were introduced). So for me it's Tobirama, Kakashi, Sasuke and Minato, in no particular order and Shikamaru, about whom I need not mention more than the Asuma Redemption Arc (whatever it was called).