The right answer is V2.
There's my hint now everyone say V2 (joking, you can say other things)
I also really like the appends for V2, especially her sweet voice, I think it sounds so nice and perfect for cutsey songs. I also really like vivid, and solid sounds amazing, but at the same time solid doesn't sound quite like Miku to me.
V4X is also special to me, since I think most Miku songs I like are probably V4X, but I still prefer V2.
A lot of modern songs use V6 and I just... don't like V6. The weird static sounds always throws me off