All the schools have their value.
The specialist school you may be in is less important than what you are giving up.
The only two that are really hard to do without are Transmutation and Abjuration.
Transmutation -- It is too painful to give up Haste and the huge array of buffs.
Abjuration -- Wizards, typically being physically weak, can't afford to give up these excellent defensive buffs. It is very annoying not to have access to Dispel Magic.
Arguably Enchantment or Illusion are the schools with the most potential power. In practice, you can live without.