I'm not an expert, and I don't know what the 'best' is, I just know what I like. You'll do your own sampling and determine the same. There are a million vanillas out there! Hermessence Vanille Galante is heavily discussed these days, and would be worth a try as it is different. It isn't heavy on vanilla at all, and isn't as sweet as so many of the more gourmandy vanillas like the CSP range. CSP has a plethora of vanillas (Vanille Extreme) and her combos like Vanille Pithaya, Vanille Abricot, etc., etc.
For something equally different, Micallef's Vanille Aoud, which is gorgeous and warming.
There are also houses that are exclusive to vanillas like La Maison de la Vanille.
Have fun sampling!