I use dictionary (dot) com for finding definitions and it is quite good, but lacking syns and ants...it would be great if anyone could refer me to an online dictionary, resourceful in this matter.
As always, 'the best' depends on the purpose for which you want to use it. But here a few suggestions:
Simple thesaurus: Press shift+F7 in Microsoft word
Online: thesaurus.reference.com
Personally, when I really want to be specific, I go to etymonline.com which gives me the roots of words (this in itself sometimes helps me remember a better word to the one I start with). From there, there are links to dictionary.reference.com which them has a link to thesaurus.reference.com and a lot of other language and writing based goodies.
I like the Moby Thesaurus at http://www.dict.org/bin/Dict It's old but very complete. There are several dictionaries as well as the thesaurus available at that site.