I like F1 history a lot, so I often find these kind of questions intriguing. But as Nitrodaze says, any comparisons are only going be relative.
I would take Hamilton above Mansell. Mansell, while spectacular, was a notch below Senna and Prost in terms of overall performance. Anyone else there in that timeframe? Hunt, D. Hill? Hamilton still looks more impressive. However, I can't put Hamilton above Stewart. Stewart in his prime was a clear benchmark driver of the field, which puts him into an absolute elite group of all time drivers.