The majority of my gymnasts love floor the best, I have a strong emphasis on floor, as tumbling and handstand etc forms the basis for everything else, so my gymnasts tend to be at a higher level, skills wise on Floor than other apparatus in general.
In general Beam is the least favourite. I believe at least part of this is because they perceive that they are not as good at it, a cartwheel on beam has the same FIG value as a back layout on floor. But it feels to them like they are a lot more advanced doing a back layout on floor than a cartwheel on beam.