Which Gun addon is best for DarkRP?

Try the Modern Warfare customizable weapons. https://steamcommunity.com/workshop/file...2526457139
Alternatively, if you are a hl2rp server you might like the Combine Weaponry. https://steamcommunity.com/workshop/file...=686339464
If you want more weapons, use the ArcCW GSO weapons. https://steamcommunity.com/workshop/file...2257255110 (requires https://steamcommunity.com/workshop/file...2131057232)