Greatest Test Driver ever? And why?

[herowassenna]

The thread about under performing F1 drivers has started mentioning test drivers as under performing drivers.
I figured I should start a thread about who people reckon is the greatest test driver and why.

I could start by saying that Senna was the greatest or MS or Prost or even Alonso, as they all developed their team's cars to a level where they could win races. I don't see that these are strictly test drivers in the truest sense of the word.
Senna developing Michelin tyres whilst at Toleman, or helping to refine the performance of the Renault engine whilst at Lotus was simply him improving the car that he had. Same with Prost during his career and MS during his, pushing the Ferrari team in the correct direction to achieve dominance never seen before.
Alonso also spent a year testing Benettons before his signing into the Renault squad for 2003.

To my mind, the greatest test driver, who was promoted to a F1 car because of his ability was Damon Hill.
Mark Blundell, his predecessor, never achieved anything in F1. Whilst Hill won 3 races in 1993, nearly won the championship in 1994 after being gifted 4 races due to Schumi being disqualified after the British GP black flag. An awesome drive in Suzuka 1994. His car failed him badly during 1995 and finally won the championship in 1996.
1997 took Arrows from an embarrassment at the start to practically winners by Hungary. The car had improved so much, that journeyman Diniz qualified in the top 10 at Spa.
1998 he turned Jordan around and won their first race at Spa, and 1999 his work carried Frentzen almost to the WDC. It was sad watching Hill's decline in 1999, he hated the grooved tyres and couldn't wait for the season to end.