When upgraded with the best mods armalites are better than any melee weapon you can get.
Important note the best weapon mods cost a lot of mission points. So you are talking doing a lot of missions to get the best mods. But they are worth it.
At lower levels, melee weapons like the diamond bladed knife tend to be better. Later, guns tend to be stronger, especially when you have weapon mods for them.
Guns have to be reloaded fairly often, and especially for low level players, the turns you lose to reloading offset a lot of the advantage of a gun. Even at higher levels, many people will start the fight with their gun, but instead of reloading when they are out of ammo, they switch to their melee weapon.