There is no single winner. Use a format portfolio: modern delivery for capable browsers, sensible fallbacks, and SVG for trusted vector artwork.
Match the format to the pixels
Photographs tolerate perceptual compression. Logos and UI captures punish ringing around hard edges. Transparency rules out JPEG unless you intentionally add a background.
Delivery is more than file size
A smaller file can still be a poor choice if your audience cannot decode it or if generating it consumes too much server CPU. Responsive dimensions, caching, and lazy loading often matter as much as the codec.
A practical default
For a new web project, WebP is a strong general default, JPEG remains a safe photo fallback, PNG remains useful for exact graphics, and AVIF is worth measuring on high-traffic photographic assets.