Character build questions are inherently subjective, but given enough parameters, it's not difficult to tell which criteria to apply to get the 'best' build.
Should Character build questions be CW or not?
I have issues with using CW as a device to allow subjective content on no grounds other than the fact that the content is subjective. Putting that aside, build questions (and consequently, moveset questions) come in a lot of varieties, which I think should be analyzed separately. Some should be perfectly fine without CW, others could be CW to satisfy the naysayers, and some should just be closed as S&A probably for eventual deletion. I'm not going to bother with the obvious closers.
In most games, there's a "core version" of every build - which lets us field questions like "What is the X build for my class?" or "What abilities are necessary for the X build?". There will always be variants that different players of the same build may opt to use, but there's going to be the core elements of the build that define what that build is (otherwise, it wouldn't be called a build). It either consists of specific abilitys/skills and/or/ equipment/items that are invested in, or there is a general pool of abilities that the build focuses on with varying amounts. That tends to be very specific to the game. When it comes to variable investment opportunities (i.e. do I need full skill points in Lightning Fury), it's probably wiser to avoid hard numbers and just state focus areas. I think these questions are fine without CW.
Consequently, "What is the purpose of the X build for my class?". The very point of a build is to focus on something, and the point of that focus is to serve some purpose in the game, such as crowd control or tanking or long range artillery. Generally speaking, it's very indisputable what the purpose of a build is, so these should also be fine without CW.
Not everyone wants to just know the cores, though. We might see something like "What are the popular variants of the X build?". And here will trip up a lot of people who see these, because the very word "popular" implies heavy subjectiveness.
And if it is nothing but a popularity poll on what is "in" for a specific build, it could be CW. But instead of taking the words at face value, if the question focuses what is common amidst experts, there are solid answers to these. The very reason builds exist is because they are commonly used in high-level play. Naturally, as the metagame for a particular game evolves, these can change, but I would rule these kinds of questions as fine being without CW. For the duration that list questions are under scrutiny, I am withdrawing this portion of my answer.
Moveset comparisons like "How do these two abilities compare for the X build?" are also border cases, but not in terms of CW. There are going to be two scenarios we result in with this kind of question - either actual game statistics does not yield an answer, or it does yield an answer. In the former scenario, it then becomes entirely subjective and should be closed to avoid the resulting discussion. In the latter scenario, there are 3 possible "correct" answers to any comparison of two abilities, and they're always solid so I think these are perfectly fine not CW. Either the abilities are functionally identical, one ability is clearly superior in the vast majority of scenarios, or the abilities are evenly matched but suited to exclusive scenarios. Note on the latter scenario, the lack of a clear superior does not make the question, or the answer, subjective. Pointing out the differences in the abilities and which scenarios each ability is better for is a perfectly valid answer to a perfectly valid question. It may not yield a concrete answer to "Should I pick A or B", but the answer being a toss-up is not something I'd consider grounds for CW or closing.
One last one, which I hope we don't see often, can be "Does this setup of my character count as the X build?". This might strictly be answered by scenario 1, in which case the question is better asked as what constitutes the build instead of focusing on your own character. If knowledge of the build is already present and/or the question is just asking for a character rating and opinions, then it's very much a candidate for closing. It's both localized to that user and subjective if the answer to scenario 1 is insufficient.