While I wouldn't feel qualified to say someone is the best since I am a SF neophyte compared to most here, I think some very good SF writers are:

Orson Scott Card (Ender's Game and series)

Kim Stanley Robinson (Mars trilogy)

Clarke (Rama series through book 3, I never got around to the fourth, just lost interest)

Dan Spires (Mit-Ro-Don)

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Off-topic to the thread - I recently picked up Heinlein's Stranger in a Strange Land from my Library. Its very interesting reading SF from the early '60s. Much like watching the original Star Trek series, where all the women wear hairstyles and clothing from the era it was created, I noticed in SIASL that nurses of the future wear capes and women still darn their husband's socks. Not to mention, Mars is inhabited. Its very interesting, but very dated material. Altogether the book moves pretty slow, so it may soon get returned unfinished.

I believe Clarke's Rama was written about the same time, but somehow I barely noticed it.

-Steve G.