noun, na-adjective, noun which may take the genitive case particle 'no'
exceptionally effective; outstandingly effective
Used to describe something that works remarkably well, often in advertising or product descriptions.
This new medicine is exceptionally effective.
I found a detergent with outstanding effectiveness.
効果的 means 'effective' in a general sense, while 効果抜群 emphasizes that the effect is remarkably high or outstanding.
Compound of 効果 (effect) and 抜群 (outstanding, preeminent).