Well best is very subjective and highly dependent on many factors revolving around your experience and personal taste. 3D and 2D artistically are just different not better or worse than the other. Though some things are easier to do in one or the other. So artistically I would say that's totally up to you.

Given that this is the beginners forum and the nature of question you asked, I am guessing you have fairly little experience in game programing. If that is true then I would suggest 2D would probably be the "better" option for you at this time. 3D game worlds add a LOT of extra issues that can totally overwhelm a beginner. I would say if you haven't made a 2D game, then you should start there. 2D games still teach you about much of how a game is built and there won't be as much to go wrong and interfere with learning.

That being said if you develop a specific desire to learn about 3D games then go for it. Just know it may be worth working up to and not jumping right into.