At the risk of starting a flame war, I'll say that I think Lord of the Rings is better, if only because it is sufficiently epic in scope and narrative, and it has withstood the passage of time exceptionally well. A Song of Ice and Fire is, by way of comparison, more of a soap opera which is epic with respect to length, but it's still unfinished, and fifty years from now, I'm not sure it will be revered as Lord of the Rings is currently.
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