David Miliband seems to be the first modern choice that comes to mind. He's the only real statesman-like figure I can think of in the Labour Party today. And the only one with any credible vision for "post-Blairism" - that being a centre-left Labour ideology that didn't veer back into old socialism. One wonders if he would have had the political weight to muscle the Conservatives out of power had he become leader. I think people, particularly those on the right, greatly underestimate the extent to which a centrist Labour Party could have posed a threat to David Cameron. A simple fact in British politics is that the public at large, whether rightly or wrongly, views the Labour Party as much more compassionate and in touch with average people than the Conservative Party. That means that if a Labour leader can do as Blair did and add a strong dosage of economic competence and pro-market policy into a soft-left agenda, the entire ball game changes. Suddenly Cameron's blend of flimsy "compassionate Conservatism" looks quite weak. And I think a lot of the very real issues, particularly of Osborne's incompetence and failure to address economic woes, could have come to the forefront.
Looking a teensy bit further back, both William Hague and Michael Portillo could have been effective Prime Ministers. The former definitely came to power in the Tory Party far, far before his time. Had he played the game more safely, and waited perhaps until 2005 to run for the leadership, we might well be talking about a Prime Minister Hague today. He had that same kind of political skill and know-how that meant he could have fought his way to the top, but battling the Tories back into power from 1997-2001 was simply an impossible task. Portillo, to clarify my point, probably would have been a better Tory leader than Prime Minister, though I definitely think he had potential. He is much more socially liberal than the broad mass of Tories, even today, and may have been able to combine modernist social policy with moderate-Thatcherite economics in a way that would be far preferable to David Cameron's "believe in both everything and nothing, appeal to all camps and none" approach.
Paddy Ashdown, as many have mentioned, could have been a very formidable character in an alternate universe. He was certainly belied by the small-scale status of his party. A Lab-Lib pact under John Smith from 1997 onwards may well have resulted in Ashdown becoming the more popular and powerful of the two men. And it might have been the only route he really had to become Prime Minister, perhaps through a winding path of both collaboration and intrigue that could have seen him head-up a more permanent left-of-centre Liberal Labour coalition, or even a merged party.
It may also be worth considering the other scenarios in which Gordon Brown could have become Prime Minister. After all, he probably would have done had some kind of circumstance prevented or discouraged Blair from running after John Smith's death. We know that a post-1997 Brown government would have been marginally more to the left than Blair's, but where else might it have differed? The most obvious way would be how it might have established the legacy of Brown. By 2007 the man was battered, bruised, and handed the reigns of government at a time that initially seemed favourable, but would ultimately transform into the worst possible scenario. But if he had taken power in 1997, he would have been at the peak of his mental and intellectual strength. Could he have weakened Scottish nationalism, and perhaps even the rise of the left-wing Labour resurgence by administering a more gradual shift to the centre?