Nah, it is and will always be Marshall, he could do it all, quite simply the perfect quick. Sub 21 average over 81 tests is quite incredible, more than 4.5 wickets a test is also amazing considering the quality of bowlers he was competing with for wickets. Understood how to adapt to conditions better than perhaps any bowler ever. Shaun Pollock once said that he learnt everything he knew about bowling in about 3 weeks from Malcolm Marshall and that it was largely responsible for his success. In terms of stats his 'worst' opposition was Australia against whom he average just 22.51, his best opposition was England against whom he averaged 19.18, that's incredible consistency.
Mcgrath was doubtless a great bowler, but IMO Marshall was just a distance above the rest. For example Mcgrath averaged over 27 against South Africa, suprising considering he only ever played them in pretty favourable conditions, he also averaged over 25 against New Zealand whom he only ever played in favourable (sometimes very favourable) conditions. Also he averaged over 31 in Pakistan where his lack of pace and/or reverse swing seriously reduced his effectiveness, Marshall on the other hand averaged just 21.45 in Pakistan. Also if you look back to the 2000/01 series in Australia where New Zealand basically looked to leave as many balls as possible against Mcgrath they really neutered him, he averaged over 65 (just 5 wickets in 3 tests) with the ball, by doing this they showed with real application against Mcgrath you could see him off, they didn't clam up too much either as Mcgrath's economy rate was 2.79. Marshall's extra pace and variations meant that this wasn't possible against him. Basically my point is while Mcgrath was a brilliantly bowler whose strenghts were accuracy and bounce, Marshall had more than this and combined with his astonishing cricket brain made him effective everywhere against everyone.