ET, hands down, is the BEST. I have no clue about the worst....

War of the Worlds followed by E.T. was the best Scifi he did. My favorite Speilberg film was Scheindler's List followed closely by Saving Private Ryan.

You know Spielberg only really ever did one fantasy film, the critically panned Hook, and other than that he hasn't really explored the genre. Maybe Tintin will change that.
: )
In terms of his science fiction films, E.T. the Extra-Terrestrial and Jurassic Park are great films. I also really liked A.I. Artificial Intelligence despite its flaws (script needed to be revised, tonally it was uneven, and the actors were very much miscast).