Glass lenses are the most resistant to scratches. However, nothing is scratch proof, not even glass.
Most lightweight lenses (High Index, Polycarbonate etc.) are made of a dense materials (the higher the index the denser the material), which requires an additional coating to improve its resistance to scratches. The denser the material the harder the coat is required. One thing to keep in mind is that the harder the coating the less the lens will accept tints
Some A/R coatings are susceptible to scratching easier, however A/R coatings can be stripped and reapplied eliminating all minor scratches. Stripping and reapplying A/R coatings does carry a small risk. During the stripping process lenses may be ruined (becomes foggy and/or completely opaque) in the process. This process will also not eliminate deep scratches that are beyond the coatings and into the actual lens material.
Premium A/R (Anti-reflective) coatings offer an added layer of scratch coating that repels smudges, dirt, water spots, streaks, and things of this sort.
Premium A/R coatings are more durable to scratches than regular conventional coatings and usually cost more for the technology required to offer this durability and non-glare performance.