This is a tough one to pick out, Warne and Murali were both great spinners and although I haven't seen some of the names of the past, it's clear that statistically the two only have each other for competition.
In my personal opinion, Warne was the better of the two but only slightly, it's like comparing the batting greats of the last 20 years, namely Tendulkar, Ponting and Lara, at his peak Tendulkar was the best, the greatest batsman who ever lived, Ponting was probably the grittiest and Lara the most stylish.
Anyway, back to spinners, over the last 3-4 years Ajmal has been the best, he spins the ball more than anyone else and he is just a joy to watch. Swann isn't too far behind though. Narine has a lot of work to do to be on that level.
In terms of ODI cricket, no one's mentioned a little known man...Shahid Afridi