Is this pre or post the supposed 15degrees of arm 'deviation'?. Some of the 'bowling' I've seen lately is just out & out throwing. Changing the rules & sidelining the umpire has led to the huge increase in this type of bowler becoming successful.
No contest, Bishen Bedi. To watch him drifting and turning with 2 close in fielders either side of the pitch was a pure delight. Deadly Derek Underwood was unplayable on his day, but needed conditions to suit him in order to dominate. Vettori's very good also, but Bedi was the classic off-spinner. Gary Sobers used to bowl them pretty well. Ray Bright was pretty good until too much bowling to world class batsmen in WSC saw him flatten out his bowling to suit one day cricket.
Why don't we see more left arm spinners? Because they all seem to take the soft option of bowling othodox around the wicket and push the ball through. Whereas this is okay for the shortened form of the game, they will never gain success at Test level.
When I was a boy, I loved watching David Sincock, a left arm chinaman bowler for SA. He could turn the ball like you wouldn't believe, but unfortunately lacked consistency with control. Very successful at Sheffield Shield level but struggled in Tests.
From my 45 years of watching test cricket, it would have to be Derek Underwood as he was nearly unplayable
when the conditions where in his favour. From the purist angle, some might say that Bedi was better because he flighted and drifted the ball, whereas "Deadly Derek" wasn't truly a spin bowler as he bowled at almost slow medium pace at times and could cut and even swing the ball, as well turn it.
Actually another topic you could discuss would be who was "the best Test keeper" to slow bowling ... and if you watched Alan Knott in tandem with his English and Kent team mate, Underwood, there would be no doubt who the No. 1 keeper to slowies would be.
In recent times Vettori would be the best, and from the vault, Rhodes and Verity have the figures and longevity to say that they must have been something special in their day.