What about Mikka Hakkinen?  
I think he deserves to be on anybodies top 5 drivers of all time list.  I loved him just for the awkward silences his single syllable responses in press conferences led to.  

Problem with picking a "Greatest Driver Ever" is that the cars and tech are so different from one era to another it's hard to really tell.  Having said that I think it's telling that in a Top Gear short film they made about Senna last year;
 -Michael Shumacher, Lewis Hamilton, Mikka Hakkinen, Martin Brundle, Mark Webber, Fernando Alonso, Felipe Massa, Rubens Barichello, David Coulthard, Jarno Trulli and Nigel Mansell-
are all on record on film saying Senna was #1.    Oh and Nikki Lauda too  (just noticed movie cover art  

 )