I've been playing NV since a couple of months after it came out, and I currently have all the DLCs. One thing that I have noticed, which was also present (though much worse) in Fallout 3, is that many of the weapons in the game are superfluous. This is especially true in the DLCs.
Basically, I have found that a Guns-oriented character is best off with the Gobi Campaign Sniper Rifle and the 12.7mm SMG.
The Gobi Campaign Rifle has incredible damage (especially on a critical), reasonably quick reload time, excellent armor penetration, rapid fire and not-too-explosive recoil. It has some of the best Hand-Loaded ammunition, the .308JSP as well as easy-to-obtain armor penetrating rounds for robots and other heavily armored beasties. It functions, not only as a sniper rifle, but also adequately as a point-blank rifle (I have murdered plenty of death claws spraying GCR fire at them while running backward, they're so big it's impossible to miss...)
The 12.7mm Submachine Gun, on the other hand, has a very high rate of fire, decent reload time and magazine-size, enough damage to get through even most armored enemies and thousands upon thousands of rounds can be obtained from Contreras and Gun Runners.
There is basically nothing else, even in the DLCs, that competes with these two guns for efficiency of blowing peoples' heads off. Some of the weapons, such as the .45SMG, the LMG and the Anti-Material rifle, do offer viable alternatives. However, the .45SMG does less damage and the magazine-size is rather irrelevant since most enemies are encountered in small groups and "reloading in the heat of battle" isn't very dangerous at all. The LMG fails to compete simply because of spread, relatively weak per-shot damage and poor armor penetration. The Anti-Material rifle does amazing damage, as is a great rifle for boon, but its slow firing speed, insanely expensive and rare ammo and crazy recoil give the Gobi the edge.
Other competitors would include the Shoulder-Mounted-Machine-Gun. A nice weapon, but it actually fires rather slowly, has inferior per-shot damage to the 12.7mm SMG and is incredibly expensive to repair.
I think by this point I've used or seen used practically every unique weapon in the game, but I still don't see any competition for the GCR and the 12.7mm SMG as the ultimate kill-spree weapons, especially when combined with the relevant perks. The Gun-Runners Arsenal, while introducing new weapons, actually exacerbated this problem by introdicing the 12.7mm SMG [GRA], which is even more over-powered than its vanilla brethren.
It seems like, if I want to play efficiently, I will more or less ignore all the DLC weapons, as they just aren't worth the ammo, caps and weight; especially considering how trivial it is to get a 12.7mm SMG and GCR fairly early in the game. I'd still be using the same 2 guns I used on my first character!
Not really complaining here, just pointing out that in the numbers game - especially in VATS - a lot of the newer weapons fail to offer a real alternative to the old-standbys.
I also find explosives useless 99% of the time, since it's usually just easier, faster and cheaper to just VATs everyone in the face from half a mile away. The damage and splash-radius of explosives in this game is underwhelming, to put it lightly (a rocket launcher that can't kill everyone in a 10' trailer is a lame rocket launcher).
I've found the same is true of armor. There are some good light-armors in the DLCs, but for medium-armor users the Combat Armor Mk.2 is the best except for the unique Elite Riot Gear, and for heavy armor most of the Power Armor is nearly identical.