is "aller" essential here?
Not really, but it's an intensifier. Which do you like bestest?
...Like no one ever was?
The translation "Which do you like best of all" should be acceptable (but I didn't try it!)
I agree and didn't see your post when I posted my comment (unless we posted around the same time).
It is accepted.
If you just wanted to say "which do you like the best?" would you simply omit aller?