What is an all-time great (ATG)?
To me, it is someone who is best of the best, and merits consideration for an ATG XI selection from all eras (to be shortlisted if not actually enter the XI, like Tendulkar, Warne, Kallis, etc.)
Below that are great cricketers (among the best their team has produced in their country history but not quite rated best of the best, like Waqar, Dravid, Walsh, Michael Clarke, Anderson, Martin Crowe etc.).
Below that are world class cricketers (among the best when they played but didnt play long enough to be considered a great, Shane Bond, Shoaib, Ian Bishop, Saeed Anwar, etc).
Then you have frontline cricketers who are the top cricketers and automatic selections of their specific team at the time they played (like Mark Waugh, McDermott, etc.)
By the criteria, in the last 50 years, you have the below ATGs:
Australia: Lillee, Greg Chappell, Border, Warne, Ponting, Steve Waugh, McGrath, Gilchrist, Steve Smith
WI: Sobers, Viv, Marshall, Ambrose, Lara, maybe Holding
England: Trueman, Botham
NZ: Hadlee
Pakistan: Imran, Wasim
India: Gavaskar, Sachin, maybe Kohli
Sri Lanka: Murali, maybe Sanga
Zimbabwe: maybe Andy Flower
South Africa: Graeme Pollock, Allan Donald, Steyn, Kallis