Between Nuvolari and Senna for me. But some team managers might argue that the best ever would be the pay driver of all pay drivers, Pedro Diniz!
Andretti won a WDC, he won Pikes Peak, a handful of IndyCar championships as well as the Indy500 and a few podiums at Le Mans. Easily the most versatile driver ever. My top 5 are: Lauda Senna Prost Ascari Fangio Lauda & Prost knew the ins and outs of F1 like no one else. Lauda's story is incredible to say the least and he came out of retirement to beat Prost who was in his prime! Fangio and Senna need no explanation. I think Ascari is an unsung hero. Not only did he beat Fangio, but he won every single race in 1952! Had he not died his later battles with Fangio would have been remarkable!
Massa 100% sure new parts should work : F1plus Interesting take from Massa. Many said Alonso was downplaying the car last year by tempering expectations, turns out he was just being realistic as usual.
Senna...we're talking one golden lap. He could put anything on pole, even if the car would suck in race trim.
I'll take Jim Clark over Andretti any day of the week! (Andretti was/is a great driver and a great guy though! )