For me it is my homemade 15 000 ml Air-PH gun. But as it is looking intimidating and all it is not very secret.
Thinking of SUper Soaker blasters, I would choose my modified XP 310. It is small, so easy to move and pump it up. But the fact that 3/4 of its reservoir can be pumped into the PC says for its power... No one every thought it is so incredible strong! I even soaked down CPS 2500 users, without getting wet at all, haha. The advantage of speed, as the gun is so small and easy to carry, makes it one of the best guns ever!
Also my modified CPS 1-3-5 is a great thing. Pull the trigger a litle, you get a 5 X:: stream, strong and powerful. pull more and get a 5 X:: and 2x 1,5 X:: left and right. Pull the trigger ful and you get a 5 X:: in the middle, 2x 1,5 X:: left and right und 2x 5 X:: aimed over and under the 5 X:: stream. A flash flood and a CPS 1000 in one gun, controlled by pressing the trigger harder, what a great thing. Also its k-modded PC brings great power and SO MUCH capacity, hehe, nice! THe ful flash attack even holds about 2 seconds

Which makes a output of about 500 ml/s aaa
Another thing to mention is my modified CPS 1500, with all its greatness. And also the modified Hydro Cannon^^
Apart from all modified guns, and I modifed a lot, I love my CPS 2000 Mk1 and my SS 300 for all its crazy power and greatness and just their insanity

Besides all this, my XP 270 stands also quite up there, as it was the FIRST SUPER SOAKER I ever had... what a great improvement and big difference related to all the other crap guns! hahaha