I've read Shakespeare, and watched one of his plays, and I find him decent.

Adjusting for cultural mismatches and the number of authors back then, I find it probable that he indeed was one of the best of his time - possibly the best, I haven't read all the others. However, I have no trouble naming any number of modern stories that are far superior, even - I believe - adjusting for said cultural mismatches.

Modern authors are simply better, on a per-author basis. There are many reasons why this might be; more time, more schooling, better tools... and of course there are simply more of them.

If I were to name the best author in history (so far), I'm not sure who it'd be, but the authors who'd be in the running are uniformly recent. Stefan Gagne and his City of Angles, for instance, would be high on the list. Shakespeare wouldn't figure on it.