Some posters are seriously underestimating Imran's worth as a bowler, he was a genuine all-time great up there with Lillee and Marshall in the highest pedigree. Imran was a pioneer of reverse swing, bowled mostly on pancake wickets, and had a better record against the mighty West Indies line up than any other pacer of the era. Miller had only 170wickets in 55 matches and you seriously suggest he was better than Imran in this category? A comparison with Ray Lindwall is more suitable.
Miller was arguably the better batsman, but not by the wide gap some people are suggesting. Imran had a slow start career-wise, but managed to average over 50 is his last 50 tests, no small feat when you consider he batted so low down the order. He could adjust his strokeplay to almost any situation, and played purely as a batsman when he was injured as a bowler.
Finally, he wasn't just a captain, he was a leader. Landmark victories in England and India, never losing to the West Indies, winning the World Cup, etc. All this with the most fractious side in world cricket.
Don't get me wrong, Keith Miller was fantastic, a man capable of captivating audiences with the ball and bat. But come on, he played in the Invincibles alongside Bradman, Barnes, Lindwall, etc., you can't tell me he experienced the same pressures as Imran did to transform his side to a cricketing force.
For his status as a bowling legend, fantastic stats against the best, shouldering the lion's share of his team's responsibility and a proven track track record as captain, Imran simply rates higher. Don't just take my word for it, ask Richie Benaud, who adored Keith Miller but had to concede that Imran was the better all-rounder.