Just a small correction, but Ratzenberger was Austrian, not German
There are a number reasons as to what might've caused Senna's death. This famously led to the whole incident going to court as several Williams people like Adrian Newey were under investigation. When a death of a driver happens in Italy, it gets taken to court. The whole saga lasted for three years until an eventual conclusion that no one at the team was proven at fault for his death.
What didn't help in his crash was the black box, that records all the data logging for the car was destroyed in the accident, so that didn't help matters.
Senna was definately one of the best of all time, but it's debatable if he was the best. I feel a lot of people jump onto the bandwagon a little of Senna because he was a very charamastic guy who touched the lives of so many around the World that gone too soon. People tend to romance a bit when someone is taken too soon I've noticed. But amongs the great drivers, he did a lot of bad things too. he did many things that were very questionable, but yet seems to get away with compared to say someone likes Schumacher antics.
It's impossible to compare eras but to me, someone like Jim Clark is possibly the greatest driver of all time. He could've had so many more wins if Lotus wasn't so crap with reliablity over the years. Won the Indy 500, BTCC champion too, but what I liked about him despite not being around when he raced, he seemed like a totally normal guy. When he wasn't racing, he was a farmer in Scotland. On his grave, he also mentions that part, as well as being World Champion.