It is tied between the Fatman and the Tesla Cannon.
Actually,is the Xperimantal MIRV,but a really best weapon to use is the Gatling Lazer,Specially if one of ur tag skills is Energy Weapons/Big Guns(Can't remember if is EW or Big Guns that affects Gatling Lazer)
Per shot the gauss rifle and alien blaster both do 100. The Fat Man and Experimental MIRV only do 10 per shot, but have a 1600 AOE attack which is the most you can inflict per shot. The MIRV inflicts the most damage the fastest, with a DPS of 2542. By comparison the Tesla cannon has a DPS of 189.
It gets very techincal. 'Strongest' and 'best' are not synonymous. There are plenty of unique variants of weapons that prove to be extremely potent (for example, the Xuanlong does a great deal more than the regular Chinese Assault Rifle, and has some of the highest DAM and DPS ratings amongst all guns. By far, the MIRV has the highest possible damage output, essentially being eight nukes being launched simultaneously. However, by far the best weapon in terms of handling and sustained damage would be the Alien Blaster. Its base damage is already amazing, but combined with its guaranteed critical damage bonus, it is the best weapon in the entire game. Obviously, the MIRV and Tesla Cannon have higher potential damage ratings, but are harder to handle inside buildings or metros.