Murali was the greatest bowler ever, without question
The evidence is that:
1. We have to assume that standards in sport are rising all the time, so that recent sports people are better than those of previous generations. This applies to all sports with objective measurements (running, jumping,, throwing etc), and we must assume it applies to competitive sports.
2. The most valuable bowler is a great spinner. Because a spinner can bowl more time and more overs than a quick bowler, as and when required.
3. Murali is the greatest spinner of the most recent generation. His main rival was Shane Warne, and Murali outperfomed Warne.
I did the folowing analysis in 2008, before Murali's recent decline, which was published as a letter in Wisden monthly.
v England: Murali 7.15 wickets per match @ 19.74; Warne 5.41 wickets per match @ 23.25;
v South Africa: Murali 6.93 wickets per match @ 22.22; Warne 5.42 wickets per match @ 24.16;
v India Murali 4.47 wickets per match @ 32.47; Warne 3.07 wickets per match @ 47.18;
v Pakistan: Murali 5.64 wickets per match @ 23.31; Warne 6.00 wickets per match @ 20.17.
Warne out-performs Murali against Pakistan, but Murali dominates Warne against the other three major test teams.
I would particularly highlight Murali against England - considering that Warne was always considered to bowl especially well against England, yet he was totally out-perfomed by Murali; and Murali's superb performances against India - in an era when India had the second-best batting in the world.
(India were second only to Australia, against whom Murali still managed to take a very impressive 4.5 wickets per match although at a modest average of 36 - sadly we will never know the comparison of how Warne would have performed against Australia.)
I lay considerable stress on the statistic of the number of wickets a bowler takes per match as a measure of how much a bowler contributes as an individual to test victories. In this respect Murali ranks very high throughout history - with only SF Barnes scoring significantly higher than Murali's 6 test wickets per match - and over a much smaller number of matches than Murali.
And of course Murali's basic statistic is that he has taken more test wickets than anyone else ever - which reflects that Murali's incomparably high value to his team over many years.
Murali therefore is the most recent bowler who made the largest contribution to his team (both in sheer volume and also per match) - and he therefore deserves the accolade of 'the greatest ever'.