The
SMC coating found on all K-mount lenses is great - far better than that of many competitors.
However, although this isn't really advertised, many new lenses have a ghostless coating, which reduces flare even further. This coating is found on FA* and higher-end DA lenses.
Weather-resistant lenses also feature a special coating which protects them from dust.
M42 lenses without
SMC in their names have older coatings that aren't quite as effective as
SMC, or even no coating, as canada rockies said.
In the past, pentax also produced budget lenses without coating.