Effective means impressive, in effect, or producing a result.
- The band put on an effective performance at Saratoga.
- The new fiscal calendar is effective as of next month.
- The talks were effective in gaining peace within a few days.
Effectual, like effective, means sufficient to produce a desired effect or result.
- “Effectual steps for the suppression of the rebellion.”—Macaulay
Efficacious means having the power to achieve the desired effect.
- The doctor discovered an efficacious remedy for the ailment.
Efficient means achieving results from a good use of resources.
- Some fuel-efficient cars can get 60 miles to a gallon of gas.