I think Niki Lauda and Sebastian Vettel. It's interesting because I wasn't alive when Lauda was racing, but to this date I feel that he was the bravest driver ever, and certainly a force to be reckoned with throughout his career.
Also, I'm really starting to appreciate Schumacher now that I've been watching Formula One for a few years. He "retired" just before I started watching. Now that I know how difficult it is to win a championship, I'm amazed that he won 7 of them and dominated for so long.
Fangio is another one who should be added to my list. And surprisingly Alonso, who I used to detest, I would add him if he could win one more title...

Part of my reason for supporting drivers (in case you can't tell) is their achievements.
A special mention also needs to go to Wolfgang Von Trips who I have read about. He could have been the first ever German World Champion if he had've finished 3rd at the 1961 Italian Grand Prix. He qualified on pole, but on the second lap, was killed in a horrific accident, along with 14 fans. But I think reading about him has had a large impact on my motorsports life.