Other than Feroze Rawther, I dont think anyone believes that Kapil was a greater test all rounder than Imran. I think we can safely conclude that IK was the better Test Allrounder between the two, the best from Asia, and the 2nd or 3rd best overall.
But, I will concentrate on ODIs here. I believe that Kapil was a slightly better ODI all rounder than IK. I will base my argument on ICC rankings because
1. ICC rankings take into account the quality of the opposition. A 50 against a tougher opposition will be ranked higher than a 50 against a weaker opposition.
2. ICC rankings give more consideration to World cups than they do to bi-laterals
Bowling
The following observations can be made from the bowling graph
1. Kapil Dev's rating rarely went below 750 after 1982, whereas IK's rating crossed 750 only once in his career in 87-88.
2. Even though close, Kapil was consistently rated higher than IK as an ODI bowler throughout his career. I understand that ICC rankings are not everything, but being ranked higher than a player for most of ones career tells a significant story.
3. Kapil's best ever rank was 1, whereas IK's was 4. I wont go too much into this, as a player can reach rank 1 just by having an unbelievable series or 2. So, this could be anomaly. But, Kapil being ranked consistenly higher as a bowler throughout their careers can lead to the conclusion that Kapil was slightly a better ODI bowler than Imran.
Batting
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1. After 1982, when both these players came in to their own, Kapil was rated higher than IK for most go their careers. Between 1982 and 1989 Kapil was clearly the better bat between the 2. Only at the end of 1989, when Kapil's batting declined, and Imran's batting came into its own when Imran overtook Kapil as the batsman. So, during their careers, Kapil was a better bat for 7 years, and Imran for 3.
2. This is also visible in the best ever rankings. Kapil's came in 1983, whereas IK's in 1990.
3. Again Kapil's highest ranking is that of 6, whereas IK's is 10. But again, I wont go too much into this, as one can reach a higher ranking by having a great series or 2.
Again, in conclusion, Kapil ranked consistently better ODI bat than IK for most of their careers. So, Kapil can be considered the better ODI bat between the two.
Overall All-rounder
Given the above bowling and batting rankings, the all rounder rankings should show a similar picture. Kapil being ranked higher between 82 and 89, and IK slightly higher between 1989 and 92. And thats what the All round ratings show
1. Kapil was never rated below 500 between 83 and 89 (other than a small blip in 87). Between 89 an 91, Kapil was rated just below 500, after which he had a steep decline.
2. Imran, on the other hand, was never ranked above 500 as an all rounder. He was easily rated below Kapil for a period of 6 years. For 3 years from89, until his retirement, IK was slightly better.
Again, looking at the rankings we cannot discount Kapil's being clearly ranked above IK for a significant period oh their career. This clearly shows that Kapil should be considered a better ODI all rounder than IK.
Now, I understand that rankings and stats are not the only means of measuring players. But when rankings show such a consistent difference between the two players for a majority of their careers, it cannot be ignored.
Also, please note that I do not believe Kapil to be the better test all rounder. IK was the better test all rounder and the better over all all-rounder between the two. But, only for ODIs, Kapil was the better all rounder.