I suppose for ruthlessness you need to consider how each driver went about trying to win. Senna was ruthless in every race in some respects because he simply drove flat out, and there was that occasion where he took out Prost. Prost used other tools at his disposal (i.e. great knowledge of the politics) to be ruthless, and to do whatever it took to deny Senna the title. Schumacher was similar in some respects but was able to build a strong team around him, reinforcing any weaknesses he may have shown. We have seen drivers change their mentality too. Hamilton of old was absolutely ruthless when one-on-one with another driver (usually Massa), but as we saw in 2012 he is beyond that now. To a degree Vettel had been the same on track, and then Malaysia came along. That was a big sign of ruthlessness.
Every driver wants to do everything they can to win, however some of the things they do are not morally right, nor are they in the rulebook. I don’t think there’s a straightforward answer to this question.