The most powerful weapon known to man is a fist. The extrapolation of this is that the most powerful robot weapon is a robot fist. Then, you put that fist on the end of a rocket, and make it an arm.
The Rocket Punch is ridonculous for the obvious reason that a rocket fired fist is in no way going to be a useful weapon. It might distract but it removes the mecha's hand on one side, making it less versatile. The surprise factor is not likely to be worth the cost of losing the hand actuator in question.
Big Ass Robot Drill!
A shiny cone, with the spiral looping around the outside, the cartoon drill. A metaphor for male sexual dominance? An easy to draw weapon? Sneak attack from underground? The big ass robot drill shows up regularly.
The big ass robot drill isn't even right for a normal drill. They aren't cone shaped. There are hand tool drills that are straight and have grooves to remove the drilled out material (something the cone drill lacks) and industrial drills are flat, and are wheels set with grinding and cutting teeth to chew through it substrate material like a giant metal lamprey. For a drill to work, the material being penetrated has to be still, and not move away from the drill. The big ass robot drill is just a big cone shaped punching tool.
Overly Accurate Giant Sized Robot Weapons
Sword. Axe. Katana. Nunchucks. Switchblade knife. Box cutter. Take any mundane weapon, then make it 100x bigger and give it to a mecha. It's time to brawl.
There are two massive conceits here, one is that ultra-sophisticated robots are going to engage in hand to hand combat with each, the other is that in these fights, the best weapons are going to be giant versions of common melee weapons. One of the problems sword makers have is making a blade of sword size that is strong, light, and durable. It's not an easy task. Now, making a meter long sword is a problem, then making an 8 meter sword is going to be an even bigger problem. One of the most jarring scenes in Neon Genesis Evangelion is when Asuka and Unit 2 deploy a 'progressive knife' which is just an eight foot long red box knife that has the extendable blade that you break pieces off as you dull them.
Additional Ideas (1)
Ridonculous Mecha Guns
The mecha revolver is a short barreled pistol that fires large bore rounds. The revolver has a simple reload mechanism, is highly reliable, and generally not integrated into the mech's combat computer. The weapon is generally a surprise, and most are loaded with exotic rounds or armor busters, generally for point blank combat with enemy mecha.
The SMG for people is an automatic weapon that fires pistol rounds rather than rifle rounds, giving high rate of fire at the general cost of accuracy and range. The Mecha SMG does the same, firing low velocity rounds rather than high velocity. The main use for these weapons is for mecha who are going into combat with non-mecha foes. When fighting infantry and power armor units, a linear gun is not effective, but a rapid fire 50mm gat with explosive rounds is devastating, if ammo hungry.
The shotgun is a short range area attack weapon. The mecha shotgun fires a battleship sized cartridge that can flatten a building, shred an entire infantry platoon, or turn the side of a dropship into a colander. At range they are not effective, and have almost not armor defeating power.
Like a normal grenade launcher, the mecha grenade launcher is just bigger. The average (medium) mecha grenade launcher lobs a 250 kg bomb that explodes as a light artillery strike, or can fire gas, smoke, chemical, or other rounds at medium mecha range.
3 to 7 barrels, firing 30mm to 75mm autocannon rounds. The weapon is enormously heavy, and can delivery a devastating amount of firepower at the cost of sucking down literal tons of ammo.