For me, there's 2 weapons that I would want.
The first 1 is the OMNIWRENCH. Is the coolest wrench in the universe, and the most useful too. In Ratchet: Deadlock, you could buy the Inferno Mod, which would make your wrench have fire around it. Each time you hit someone, he would turn into fire! Also, in Ratchet And Clank Future: A Crack In Time, the Omniwrench can also use magnetism to move metal objects to different places ^_^
The second one is the GRAVITY HAMMER from Halo. When I played with my friend who had an Xbox, we would play Co-op, and I would always try to find the hammer. And when I had it... HELL YEAH! I would jump onto an enemy from a higher ledge and hit them with the hammer JUST as I hit them. Instead of going in the air, they get crushed by the force of the hit. I called this technic the "Aegis Killer" technic ^_^
Edit: Oh, and also, these are my favorite weapons, but I would basically want EVERY Ratchet And Clank weapon: their are SO original