1989: Senna does his moves, he's an evil man with a Twinkie-colored helmet. Prost does it, he's a hero for finally putting him in his place.
1999: After Senna dies, his moves are always brilliant, but now Prost is a jerk.
That's not the impression I've had at all. I think you'd struggle to find anyone who'd call Prost "a jerk", even considering his move on Senna. He was very much considered very much a thinking driver - a bit like Button today, I guess - and always very quick and efficient.
As far as outright skill goes, from the races I watched as a child and the races I've seen since on video, I'd rate Prost, Senna and Mansell very closely. Senna probably had the edge on outright speed and that ability - like Schumacher had - to pull off the virtually impossible occasionally. But Prost and Mansell were very, very close, and the three of them provided some of the best racing the sport has ever seen.
Important to note on the "Would we be calling Prost the greatest had he died instead" theme: Aside from it being a bit morbid, you can't really draw a comparison. Prost had retired at the very top of his game. Senna's 94 was off to a very bad start already, and who knows whether it would have improved? Maybe if Senna had survived we would still have seen Schumacher dominate the next few years, and that might have harmed his reputation. As it was, Senna had been at the very top of F1 for several seasons and we can only judge him on his performances - and his performances had been utterly stunning, even in years like 92 when his car was sub-par.
Prost had always been at the top, but not "sparkled" as much as Senna even when both were racing together.
As for Hill in '94 - I think he did amazingly well to carry the team after Senna's death - but let's not forget he was actually brilliant in 93 too. Theoretically he could have won the 93 title - his car broke twice while he was in the lead of races
(off the top of my head - Monza and Hungary) (UK and Germany) and was ordered to let Prost pass on another race (France). He was also catching Prost at Spain before his engine blew. Those four sacrificed likely wins would have altered the points enough for him to become champion, if I'm not mistaken (I know, I know - ifs and buts). Of the races he didn't retire, he came 2nd four times, 1st three times and 3rd twice
And by rights he should have won 94 too, but we all know the outcome of that...
That said, Hill was never as naturally talented as Schumacher. Nor Hamilton or Button, I'd say, and I'm saying that as a big fan of Hill both as a driver and as a person.