This was a tricky task, I don't practically have a most used polish nor a most used colour of polish. I'm always chopping and changing my favourites and can never decide on a definite answer.
I though for a while and realised there is one polish that I use all the time, every time. My Sally Hansen Maximum Growth which I use as a base coat every single time I change my polish.
This isn't a colour as such, it has a pale creamy colour which evens the colour of my nails and hides any stains that I may have. it goes on well and somehow smooth's out the little rides I have on a few nails.
As you can see from the photo this bottle is half used, I've not yet got it's replacement, it'll either be another Maximum growth or something similar.