Best value for fill factor in index
There is no one answer for all with fill factor.  It largely depends on how your data modifications take place database-wide or in particular indexes.  The default, out-of-the-box is 0 (100) which leaves virtually no space in a page.  What this means is an insert into the middle of a page will cause a page split.

Monitor your Page Splits/sec counter and if you are seeing a high rate, then you may want to reconsider your fill factor.  But don't just blindly set that low, as you will be using more disk space with a lower fill factor.  And naturally, it will cause higher IO reads, as there is more pages to house the data.

I recommend you look at my answer here regarding fill factor.  It's a different type question, but it should give you a better idea about fill factor and how it affects you.