For me it would probably be Peter F. Hamilton and Iain M. Banks, although I wouldn't dare claim they're the best, only my favourites. Stephen Baxter is certainly up there as well, as is Ben Bova and Brian Aldiss (who is still alive, I think?). The latter primarily for his Helliconia trilogy, which is one of the all-time greats in my opinion. Many others too, I'm sure, if I started going through 30+ years of Sci-Fi reading.
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