For a spinning reel you can not beat either the Gammakatsu polypro line or Trilene XL. The XL isnt super tough but it is super limp and is thin enough that you can step up in test and still fill a spool with enough line.
I will never go back to fishing mono for floats. On the rod I use for floats I fish with I think 50# Tuff Line. The stuff casts amazing off a spinning reel but it most importantly it FLOATS. I heavily grease it with fly floatant when I am putting it on my reel and apply alot of pressure so the grease soaks in. Even with just a 8 1/2 rod I cast 3/4 across the Sky at Reiter and control my drift. It works perfectly, it is also highly visible so I can see if I have a belly in my line and how and when I need to mend. Never again will I fish mono with floats.
Ryan