That is like saying which modified mustang is the pinnacle? Cobra? Shelby? Which modified light is the pinnacle? Milkyspit? Overready?
It really is the user that makes the difference. However there are some guns that are so overwhelmingly unfair that they are banned.
Doomslayer is one that comes to mind. It is a Rapid Fire Shot gun by Ertl. Modified it reaches 100ft.
the Nerf Crossbow is like the AR15 in Nerf world. it can reach 100ft easily and has LOTS of mods for it. But it is old and rare.
The Octo-shot is pretty good from what I hear. Here is a video that was posted in the marketplace.
http://www.youtube.com/watch?v=EO36G4W2Occ
The raider is great with a spring upgrade and restrictions removed. It is an all purpose nerf weapon. I have managed to hold my own out doors against 100+ft guns. The benefit is the large ammo capacity. 35 darts is over whelming against someone who has rate of fire equal to the civil war.
However newer guns like the plusbow (DIY open source project cross bow) Has evolved into a pump shotgun with the "hopper" clip (nerfers say clips. Firearms people say magazines) with the hopper clip you can shoot as many darts as you can pump them out and reach over 100 ft. so those types of guns are drastically changing the game.
Here is a fun project that i made.
The Longshot (LS) is a pretty good platform. modded properly it can reach respectable ranges. Of course dart choice is key. The stefans help revolutionize Nerf guns. They are more stable in flight and reach farther distances.
Here is my shotgun LS. The LS stock has a charging handle on the side. i have modded it with only stock parts to make a shotgun pump version