While I am reluctant to use the term "best" as there are probably other good ones too...but, this is the gear that many top shelf astro-photographers use...
http://www.catseyecollimation.com/
I am certain that there are other sets that will work but this stuff is high quality...as normal, you get what you pay for...and these are not in the same price category as the GSO...
You can do a search for catseye or search strongmanmike's past posts here on IIS...
Clear Skies!