I voted Sebastian Vettel. When he started out, he was nowhere near Heidfeld and could not outqualify Liuzzi. The only times, when he was convincingly ahead of Liuzzi, were the two wet races in late 2007. I think Vettel's improvement has been significant since then, considering Liuzzi was not all that talented and was later beaten by Sutil.
Button however adapted to F1 quicker. He outqualified Ralf Schumacher in two of the first four race weekends. Though finished behind Ralf in both Brazil and UK race, was very close to him.
Massa I remember crashed a lot, but his speed was decent from the get-go. Scored points already in his second race in Malaysia, and added a further fifth in Spain. Plus outqualified Heidfeld a fair amount of times.
Now that's a close call. And Vettel was handicapped by getting thrown in mid-season with lesser testing. Sure enough, they all have improved since then. But feel Vettel needs to be highlighted since he hasn't been mentioned here so far.
---
Actually, should not forget Rosberg either. Though he started out fantastically well (FL in Bahrain and 3rd on the grid in Malaysia), was beaten pretty convincingly by Webber that year overall. I remember his 2007 looked a huge improvement over 2006, especially in terms of consistency.
Raikkonen obviously has improved a little bit, since he was narrowly beaten by Heidfeld.
Alonso's and Hamilton's improvement is harder to quantify. Hamilton matched the star driver on his debut season - can you do much/any better than that? And Alonso had rubbish team-mate(s), while he seemed to be punching above his weight in that Minardi anyway by qualifying 17th-19th pretty often.