It's probably 80% car performance and 20% driver skill
1991 - Senna beat Mansell. Mansell had the superior car and the only thing that scuppered his campaign was reliability.
1992 - Mansell with the most technically advanced F1 car ever.
1993 - Prost with a development of 1992 Williams
1994 - Schumacher - Rule changes banning driver aids meant the cars were pretty much similar in performance and the most skillful driver won
1995 - Schumacher again
1996 - Hill, with the best car and Schumi in a crappola of a Ferrari
1997 - Villneuve only because Schumacher was still in a crappy Ferrari
1998 - Hakkinen, rule changes again meant that the Mclaren was the superior car
1999 - Hakkinen again with the best car, though Ferrari gave them a run for the money
2000 - 2004 Schumacher in the best car and best driver