Which is better a cello, a violin, a viola or a guitar?

None of these is better than the other...they all have their ups and downs. Cellos sound amazing and really provide the base in an orchestra (while still having really pretty melodies and all), violins are fun and light-hearted, violas (in my opinion) sound absolutely GORGEOUS, and guitars are really versatile. Ask different people, and you'll get different answers. My personal favorite is the viola. =]