I would argue against polymorphs if you like to plan ahead and craft decks. Setting your own deck, instead of taking with the polymorph form is something major to consider.
I personally like Life School, with death to feint as a secondary. An ice school spells for protection (amulet or similiar). Level 58 clothing for life gives an 80% crit chance....so picture boosting Forest Lord with gargantuan (+225 dam, sun spell), and a crit booster (a star spell). This gives, on turn 3, 4000 damage to all foes...and no boss ever has up a defense beyond 50% (either ice or balance,,,,no one else does). It is a real sense of power for life all of a sudden.
So, for me, and I haven't yet played all the schools through CL, I would say Life, death secondary, boosted with sun and star.
Note: You can buy the life polymorph without any of the others necessary, and it also works well with all classes, giving awesome healing ability.