Gobi Campaign Scout Rifle is one of the best guns in the game. Lucky boasts a high critical hit chance, a faster-than-normal fire rate and has low AP requirements for VATS. This requires a lockpick skill of 75 and a trip back to the Bison Steve Hotel in Primm to acquire. Lucky is a revolver which uses the ever-so-common. The absolute best things you can do is maximize your guns and lockpick levels to get the two unique weapons: If you're specializing in guns you have a few options here. Melee is also a viable early-game option with weapons like Chance's Knife but this list would get really long if I included every category of weapon. Since Ratslayer seems to be the best gun you have so far I'm going to assume that you have not yet made it to Novac and thus will limit my answers to guns which can be acquired between Goodsprings (early-game) and Novac (mid-game).
