I think it was close between Senna and Prost, but with Senna winning on pure speed. The bloke who everyone loves to criticise - Nigel Mansell - was MUCH closer than he was ever given credit for though.
Not many people could beat Senna in a similar car bu the did on a few occasions. Barcelona 1991 being one example.
Not sure about the last part, I think the Williams FW13 improved throughout the middle of the season. I agree that Senna was the fastest driver ever with Prost coming second. I do agree that Mansell is underrated. He was very fast and a great racer.
Ayrton Senna: WDC 1988,
1989, 1990, 1991
McLaren: WCC 1974, 1984, 1985, 1988, 1989, 1990, 1991, 1998,
1999, 2007McLaren: WDC 1974, 1976, 1984, 1985, 1986, 1988, 1989, 1990, 1991, 1998, 1999, 2008