I agree it's very hard to compare eras. But one thing that's more valid to compare is the top drivers in the same cars in the same era.
In Senna's era, there were many greats. Lauda, Prost, Mansell & Piquet were his primary yardsticks.
'84-'85: Lauda v Prost at McLaren - Prost was faster but they split 2 WDCs while together. Prost seen as better than the aging Lauda at that time.
'86-'87: Mansell v Piquet at Williams - Mansell unlucky not to win the WDC in '86, Piquet won the '87 WDC on consistency, but Mansell was generally viewed as better.
'88-89: Senna v Prost at McLaren - Nearly equal, they split 2 WDCs. But Senna was generally viewed as having the edge over Prost.
'90: Prost v Mansell at Ferrari - Prost easily overshadowed Mansell.
Hence my personal conclusion is that Senna was the best of his generation, but Prost was nearly his equal, and Prost was better than the rest.
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In comparison, it's harder to gauge the current era, because there hasn't been the same level of direct comparisons in the same cars.
Riccardo v Vettel at RedBull - Riccardo outperforms Vettel, causing Vettel to leave.
Riccardo v Max at RedBull, Riccardo is often a match for Verstappen, but Max viewed as better.
So does this mean Max > Riccardo > Vettel??? Yet Vettel has 4 WDCs!?
Hamilton v Alonso in 2007 at McLaren - the two are nearly equal.
Hamilton v Rosberg at Mercedes - Rosberg was very tough on Hamilton, but Hamilton still viewed as the better.
Max v Alonso v Hamilton in equal cars? Hard to say who comes out on top, but that's a season I'd love to see!
