The way I see it, there are four broad levels of driver in F1
Greats - Senna, Prost, MS, Alonso, Hamilton (probably) - drivers that can outperform their car on a consistent basis and can just pull results out of the bag
Very good drivers - Guys like Hakkinen (actually that might be unfair as I think he was pretty damn good and MS respected him which wasn't the case with most of his other competition but I just don't think he was a great), Button, Hill, Massa, Webber who are generally well up there getting strong results and on their day are near unbeatable but cannot consistently outperform the car if it's not to their liking. Basically, the drivers in the top cars usually - they're not chosen by accident (unless they're deliberately chosen to be slower than the team's number 1 a la MS or seemingly Alonso).
Average F1 drivers - guys like Liuzzi, Kobayashi, Trulli, Kovaleinen - excellent drivers in the grand scheme of things but will never pull out more than reasonable results except for the odd occassion when things will all click at just the right time.
Drivers who've paid their way into F1 - Basically quite a few of the drivers in the new teams.