The problem with the verb “to want/vouloir” is that you cannot hear the difference. Even for a native speaker, it can be mistaken if you have to write it. With an other verb, it can be much easier to find the solution (if you have a good French of course) because you will hear the difference.

With the verb “to be/être” for example :
“C’est moi qui suis…” and “c’est lui/elle qui est…”

It is “C’est moi qui veux”, definitely.

In your sentence, qui is the subject of the subordinate sentence, and qui is a pronoun that replaces “moi”, therefore “vouloir” must be used at the first person ==>

C'est moi qui veux !

Note that qui will also je either masculine/feminine depending on what it replaces, e.g.

C'est elle qui est venue.

And qui replaces the nominal group that is directly before it, therefore you say (as mentioned by @Distic)

C'est moi la personne qui a faim.

because qui = la personne in this example

As a remark, there is no such consensus in English: it seems that you can write both

It's I who want 

and

It's I who wants

even if the first is more frequent.

But in French, it would definitely be a mistake to write

C'est moi qui veut (**WRONG**)