It is hard to answer.
The thing with Wasim is that he had a slow start to his career from 85-89. It was on the Australian tour of 1990 when he really came into his own as a matchwinner. From 1990 to the late 90s, he was excellent though and performed well against almost everyone. Then he again had a dip in form towards the end of his career.
His figures in the 90s are fantastic and comparable to any of the greats. Check it at
Cricket Records | Records | 1990s | Test matches | Best averages | ESPN Cricinfo. It was around this time he began to rated by his peers as the best in the world, either no.1 or joint no.1 with Ambrose (I can think of Michael Slater, Michael Atherton, Chris Cairns, Brian Lara, Stephen Fleming, Walsh, Donald, Ambrose, Ganguly, Kapil Dev, Mark Taylor, Ponting, McGrath among many others who have given him this ranking). If there was a bowler of the 90s, Wasim is a prime candidate.
The problem was, Wasim with his left arm swing, swinging both ways, deceptive speed, control of reverse swing and ability to produce a Jaffa at anytime, was an incredibly awkward proposition. Having watched Wasim for much of his career, I can attest to how opposition batsmen, especially openers, would just focus on playing him out due to respect and choose not to attack him, and attack other bowlers in the attack. Wasim himself was not the most dogged or thinking bowler (he himself said in his book that he often did not know what ball he would bowl next) and if the batsmen didn't fall for his bag of tricks it would often end in a stalemate. This was especially true after 97 onwards when he lost the pace and sting which would only resurface occasionally. I recall Warne mentioning this before Wasim toured Australia in 99, that the Aussies would just be concerned with limiting rather than engaging Wasim. This was a critical difference between him and McGrath or Ambrose, who would keep hammering away at the batsmen relentlessly in the corridor of uncertainty.
So in summary, Wasim was an all-time great, and while the opposition may have feared him the most among his contemporaries, this didn't always translate to pure wicket-taking. Which is why I rank him slightly lower on the all-time great scale.