So, there's two seperate questions that could be answered here.
Can you fight unarmed, using primarily punches, and wear boxing gloves? Absolutely. In fact, Unarmed is among the stronger combat specializations in the game, though Boxing Gloves are somewhat weak specifically, because they cause only subdual damage - meaning they will knock your opponent over, but not kill them. Useful sometimes, but rarely.
Can you fight in a 'boxing match'? Well, not per se, however, there are two opportunities for you to engage in a sporting fight for some caps. The Thorn in westside is home to one such opportunity, where you'll have the opportunity to fight off various monsters of the Wasteland such as Radscorpions, Giant Mantises, Deathclaws and Cazadors. Any weapon is fair game in the Thorn, though you're welcome to go in unarmed if you're looking for a challenge.
Male (and only male, the legion are nothing if not chauvinistic) Players aligned with Caesar's Legion meanwhile, have the option to fight in The Arena, located at The Fort. In the Arena, the only weapons permitted are Machete's. All your other weapons are confiscated at the beginning of a match, and you are of course free to use your bare hands instead if you so desire.
You don't say what you're looking for (most damage in one shot or highest DPS), but according to this, the Light Machine Gun has the highest DPS of a regular rifle while the YCS/186 has the highest DPS of an energy rifle.
Best Answer
Fallout Wikia lists the following locations for the sniper rifle:
There's also the Gobi Campaign scout rifle, a unique variant of the sniper rifle:
You may also wish to look into the sniper rifle's bigger brother, the anti-materiel rifle, or the unique Christine's CoS silencer rifle if you have Old World Blues. The Ratslayer, found in Broc Flower Cave, also deserves a honorary mention for being an easily accessible, scoped gun that silently shoots 5.56mm rounds.