As said, Trappist isn't a style but something like a certificate of origin and, let's say, quality standards. That put aside, it can be anything. Belgians themselves don't give a damn about style definitions actually. They only follow their traditions (from the region where they live, their monastery, their community, whatever).
So, you are going to find beers varying from very shinning blonde to dark/ruby and thick ones that almost resembles a wine, which is the case of Westvleteren 12, that one being the myth it is because it is not sold (at least officially) outside the monastery, and one is supposed to schedule and go pick it up personally with an amount limit, and everything. Off course, although I'm sure it is a really good beer (I've never had it), all that difficulty to get it undeniably contributes to its fame.
I find Rochefort 10 (a belgian dark strong ale as Westvleteren 12) one of (if not the) best, certainly my very favorite on the list, but on the other end of the flavor range, Achel 10 (a strong golden ale) is my favorite blonde/pale trappist as well.
La Trappe, which is the biggest trappist brewery, has at least twice the labels other breweries have, and is widely available on market, still has very good beers.
So, the short answer is: trappist is not a style, and there's no such thing as the best one. It all depends on what you like, so, taste all them. Even nowadays, with some new-age trappist beers popping up, there aren't still so many out there, so tasting all trappist beers is pretty feasible in short time (except for Westvleteren, off course).