From the few motorsports I watch religiously, I'd argue for Jean Alesi, Gilles Villeneuve, Jochen Rindt, Paul Radisich, Anthony Reid, Steve Soper and Stirling Moss. Oh, and Francois Cevert.
I'll come back with reasons later.
And here they are.
Jean Alesi
What can you say? As has often been said about Alesi, his heart took him to Ferrari when his head should have taken him to Williams. Always an exciting talent during his prime, it was so unfortunate that he was at the wrong place at the right time. The fact that he did so well in some of those Ferraris (1991, 1992) is testament to his skill. One pole position; Italy 1994 in front of the tifosi. The drama behind that on the VHS review is epic. And of course, that fantastic victory in 1995. Jean's 31st birthday. And Ferrari #27 won at Circuit de Gilles Villeneuve. Hollywood eat your heart out. These final two years at Ferrari 94-95 saw some very impressive drives. Even in his later years he was a steady pilot for Sauber during their difficult turn of the century period and then Prost. He was closer to the midfield whereas Heidfeld and Mazzacane were rooted amongst the Minardis in awful cars such as the AP03.
A very unfufilled talent in my honest opinion.
Gilles Villeneuve
Ayrton Senna with Ayrton Senna Syndrome before Senna. I don't think I can add anything to the unique legacy Villeneuve has amongst the F1 community.
Jochen Rindt
Written very highly of in Jackie Stewart's book, and the footage I have seen of him has been very impressive. Taken far too soon aged just 28 and to actually become a posthumous world champion with 4 races left is a staggering statistic.
Francois Cevert
Another driver in the Villeneuve mould but at the very beginning of his career. The 1973 Tyrrell was equally matched by his skills and thanks to 1970s track safety we have Watkins Glen to thank for taking this promising driver away.
Stirling Moss
The king who was never crowned. I've loved this phrase ever since I saw it caption a photo of Moss at Monaco in 1961 in an old F1 encyclopedia I had. Did everything right in 1958, but Ferrari and team orders saw the title go to Hawthorn. Then of course there are races such as the 1955 British Grand Prix and the 1961 Monaco Grand Prix. He deserved much more than the ignominaty of being arguably the best race winner to have never been champion.
Paul Radisich
And now onto the touring cars. Of all the drivers who I felt deserved a title, Radisich is right up there. A hugely talented driver and I'm certain that his loyalty to Ford cost him. The Mondeo only had an 18 month period from its introduction where it was genuinely competative. Had me moved to say, Renault, in 1995 and partnered Alain Menu during the period when the Laguna was the best car over an extended period of time, we can only foam at the mouth about how that could have gone. A huge disappointment in 1998 was the Peugeot. Always the weakest super tourer mechanically despite being the best looking, the final year of the 406 was a huge disaster for the superb on-paper line up of Radisich and Harvey. Moving to V8s was a completely different kettle of fish and he adapted remarkably well at first, finishing 4th in 2000 (I think) for DJR and cruely missing out on a Bathurst victory thanks to a puncture. Radisich is like the touring car Rubens Barrichello; hugely talented, but with less victories and accolades than those talents deserve.
Anthony Reid
Took Nissan to exceptional heights in 1998 and I'm still not quite sure how he lost the title at the final meeting in 2000. Took the fight against Vauxhall bravely and quite successfully with the MG and is someone else who I would not have begrudged a BTC championship. He was signed for a Jordan race seat in 1993 but sponsorship let him down. His career could have been so different.
Steve Soper
A highly successful touring car driver wherever he drove, but especially in Germany. Actually won the BSCC in 1983 but all Rovers were subsequently deemed illegal and were disqualified from the championship standings. Surprisingly sporadic in the BTCC but was always mighty impressive when he did drive. Infamy sees him blamed for the incident with Cleland in 1992 but there are other factors in that incident which should apportion blame to both drivers. Exceptional talent, but contractual obligations to race in Germany and later Japan prevented him from seriously competing in the BTCC and I think this cost him dearly.
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You may notice that drivers who I never saw race have much less written about them. Well.. that's just how it is.