The question is simple what do you think is his best work.
You can vote using any criteria you want like for example:
The one I like the most.
The one I laughed the most.
The one I think that has better artwork.
The one that has the better story plot.
The one that has bigger curves(or in some cases the one which curves keep getting bigger and bigger)
For more reference of his other works go here
Vote cancelling is allowed for those of you who change their mind(what if you start reading other of his works and it turns out better than you expected)