If we compare purely on the basis of bowling,
Imran Khan has played most of his matches on subcontinent wickets which do not support fast bowlers that much. Indian pitches were worst for a fast bowler, Pakistan had a bit better pitches but overall it didn't assist the fast bowlers as much as it does in Australian or English wickets.
Imran Khan has bowled in 142 innings picking 362 wickets at an average of 22.81(Economy rate of 2.54 and Strike rate of 53.7),
On the other hand Dennis Lillee has bowled in 132 innings picking 352 wickets at an average of 23.92(Economy rate of of 2.57 and strike rate of 52)
Imran Khan has also played more number of ODIs,
He played 175 ODIs taking 182 wickets in 153 innings
compared to Lillee who played in 63 ODIs and picked 103 wickets in 63 innings.
Lillee has played just 4 matches on subcontinent pitches picking up 6 wickets.
He has played 44 matches in Australia and 16 in England.
I m aware of the fact that Lillee overcame serious injuries with his determination to play international cricket and managed to become a much better bowler despite of loosing the pace he had.
But what exactly makes him a better bowler than Imran who bowled in much more difficult conditions?