Well, theoretically we can say - the best transformation was Don Bradman. He had a poor start in 1st Ashes Test 1928-29 at Gabba. Scored 18 & 1, Aussies lost by 675 runs (that too after first time a side declared in an Australian Test). So, he was dropped for Vic Richardson (a grudge that Bradman carried up to the grandsons of Vic Richardson- Chappells are Vic’s maternal grandsons) - Aussies lost again, and he scored a FC double before 3rd Test. So, he was called back, still Aussies lost conceding an unassailable 0-3 lead, but this time Sir Don scored 79 & 118 …. For next 20 years, never looked back.
But realistically the greatest career transformation that cricket had seen has to be Imran Khan. Went to UK in 1971 summer as a 17 years old high school kid only from family connections- his main purpose was to find a high school in UK for higher studies and when he almost knocked Aftab Balooch‘s head with beemer, Balooch shouted his mind regarding Imran’s talent and how he made the squad, because …….. he was batting on the adjacent net that Imran was bowling!!!
Since then, that kid stopped cricket for 3 years, went to Worcester Grammar School to Oxford to Sussex and made PAK team of 1974 - this time on merit, but still he was a military medium pacer who could bring the ball in from an angelic run up, but no out swing, hardly any pace to bounce and had no knowledge of cutting off the wicket.
But, he made a 160+ for Oxford against touring India which led Greig to bat him at 5 at Sussex and by 1976 he was a decent no. 8 for PAK (it was decent because he batted aged of Intekhab & Bari). Then went to Australia and found a foul mouth angry fast bowler who was extremely abusive on field on a very bad day of bowling (0/125 @ 5+), but came to talk in the evening about what better could be done ….. it worked - 12/162 at SCG did win a rare Test for PAK in Australia. Imran never forget that man to render his respect.
His ego was extremely hurt in following WIN series when to a Channel 9 commi’s note, Sir Gary said that -“if this no name can bowl at the speed to Lillee, then Lillee must have been warming - up with half run-up”. That vaunted ego resulted 30 wickets, bowling absolute flat out and it landed another opportunity- a ticket for Packer’s WSC. He learned lot there, converted his playboy like body into a NFL wide receiver like body …. but still, he was just a decent fast bowler, who could bat a bit.
Early 80s, after trying, God knows probably 10-12 captains, finally PCB had to make him captain, which led to the greatest transformation of career - a decade, hampered by internal politics, infighting, career threatening injury that cost best 2 years of career …. still ended with a batting average of 48 for 2500+ runs & bowling average of 18, for over 250 wickets ….. no one, not even Bradman had a decade like that.
For the worst …. several in recent times, sadly many from Pakistan. Umar Akmal indeed comes to mind, could be Saqlin as well, Mo. Zahid, Mo. Sami, Azhar Mahmoud, Salim Elahi …. But the guy who disappointed me most was another Imran, not Khan but Nazir. This guy was special when I saw him as 17 years old - may be take 3-4 years in it but still at 20-21, he played one of the best innings that anyone played against Ambi, Walshi, Rose & King - 131 in 2 sessions at Barbados. Bar Inzi, he was the only domestically developed PAK batsman who had a better back-foot game than front foot, better off-side game than leg side. And a brilliant, brilliant fielder as well. But, never fulfilled his potential and never tried to learn. No comparison now, but at times he reminded me of a Herschel Gibbs at his brilliant best.
In olden days, crickets greatest loss was probably Archie Jackson - Bradman’s contemporary and some people did rate him as good as …. But, he was suffering from severe tuberculosis and played often hiding illness - left world just at 23, with a batting average of 48, for the 8 Tests he could play in 3 years, but I don’t think this one should be considered for worst turnaround, rather an unfulfilled promise.