Hamilton. Yes, he was quick from the off, but he was making a lot of mistakes which he ironed out throughout the years. I don't remember the beginnings of Massa and Button's careers so can't judge them.
Did you see Hungary first lap last year? Or Austrian GP qualifying? If he makes less mistakes now (relative to then) how must they have been at the start of his career to be an improvement?
To me, he started out as a top driver from the first races of his career, with some mistakes but a great raw pace and he's still a top driver with a great raw pace.. so yeah.
I find this a really hard question to answer to be honest, it's a really good question.
Button impressed me more in his BAR days, than anything in Brawn or McLaren so not sure where that puts him. He's always been a smooth safe, calculated driver but also with great raw pace in the right situation (and poor pace at other times). Not sure if he's any better now (other than being more mature as a person overall). He seemed to drive a really mature season last year, but hard to compare that against a rookie teammate and to know where the car would have been with Lewis alongside or someone else.
Nico arguably was better in his first race than anything since. Set fastest lap with numerous overtakes after a first corner accident, recovering to points position and setting fastest lap in Cosworth powered Williams (while they were declining). Nico's always been a good solid driver, except maybe lacking a little speed or killer instinct that sets him apart as one of the top top drivers. There's never been many weaknesses with Nico though.
Kimi impressed me most in his McLaren days. 2007 and 2008 were hard to take as a Kimi fan, and it wasn't a surprise to me that he left the sport afterwards.
I don't remember much of Alonso's early years but by 2004 onwards he seemed quite solid and since 2005 (with the top car challenging championships), has seemed to stay at a really high reliable level year after year. Regardless of the car he is driving.
Vettel started out really strong at STR and that continued into Redbull. Mistakes or pace didn't ever seem a problem, the one criticism was that he couldn't overtake but during 2012 onwards people stopped saying that. By the end of 2013, no one could say anything about his overtaking anymore. And then last year.. hard to fit that in to everything else. Separate from the car's pace there were plenty of things, errors etc.. to criticise. Vettel had suddenly morphed into Webber.. was bizarre to watch. And also very ironic, given the years earlier. If it were before last season, maybe Vettel would be a contender for most improved? But it was only really the overtaking that improved.
So I guess that leaves Massa. If everyone keeps saying Massa.. maybe they are onto something.