It's quite close but I too tend to favour Ali. His body of work was greater and he did it over a significantly longer period of time. I think Leonard was the better technical fighter, but Muhammad was the original miracle worker.
A more complete fighter in every aspect than SRL is very hard to find. He lost the plot against Duran in Montreal, but he was still young then. His fight against Hagler is the perfect example of a brilliantly executed fight plan. I suppose the reason why Ali is my favourite is because of his ability to do what no one expected. Against Liston and Foreman he just didn't seem real, you couldn't believe he was simply human. No other fighter have ever given me quite that sensation.