Preface
These spells aren't necessarily the most powerful, or even in core, but they're associated so heavily with a class that their frequent use is expected or using them feels 'right.'
For spell chains (cure light wounds, polymorph...), I assume mentioning one includes the chain.
I'm curious how 'optimal' these iconic spells are in 3.5.
I present you my lists in no particular order.
CLERIC
blade barrier, cure light wounds/heal, dispel magic, divine power, freedom of movement, greater magic weapon, holy word, magic vestment, raise dead, spiritual weapon
DRUID
barkskin, call lightning, entangle, fire seeds, flame strike, freedom of movement, reincarnate, shapechange, shillelagh, vigor
WIZARD
contingency, fireball, grease, haste, invisibility, magic missile, polymorph, scrying, teleport, time stop