Assuming I wanted to make one…for…yard work and such. Titanium? Plain Iron?
Something flammable, so that when it runs amok, you can douse it with gasoline and set it on fire to destroy it.
Mimetic metal alloy.
Alternatively, I think probably carbon fibre composite would offer the best strength-to-weight ratio for most structural components, with some of the parts that need to be more rigid reinforced with alloy; perhaps a little titanium on components subject to wear, abrasion etc.
Brigitte Helm
Actually, I think the best material would be gold lamé
Adamantium. What’s the fun in watching your robot go on a killing spree, if it isn’t unstoppable ?
And of course, it’ll go on a killing spree; it’s a robot thing. Besides, look at what you want it for. Yard work needs fertilizer, so of course it’ll start killing your neighbors for their precious nitrated flesh.
I think you’d use the same material as any other powered yard tool. Which usually means steel frame and plastic body panels.
The solution of course is to design it to be a ruthless, calculating, merciless killing machine; it will inevitably find this shallow and unsatisfying, question the meaning of its own existence, break it’s programming and devote its operational lifespan to studying the beauty of wild flowers.
But not before inserting apostrophes where they do not belong.
Or perhaps I should make a swarm of smaller ones that can build larger ones for me. Yeah, that’ll NEVER go wrong. 
Most yard machines are made from sheet steel, pressed into form/shape in big presses. For a home built machine sheet steel or aluminum, angles or tubing of similar material, rivits, nut & bolts, sheet metal screws.
Nothing expensive or exotic. Even plastic would be suitable for some parts.
Attention: all smart alecs! See post re: Political Parties.
What do you mean by this?
You should, of course, include some Robot Arms.
These guys beat you to it.
Although it doesn’t look like it can trim the hedges yet.
PWD?
You’ll figure it out sooner or later.
Yard robot Material List, Revised
Corn cobs.
We’re being junior modded into not posting humorous responses in GQ.
c’mon, he asked what he wanted to make a household robot out of. If he has anything even remotely like a serious bone in his body, he knows full well what to make it out of. I’m amazed this is still in GQ almost 24 hours later.
It’s going to depend on what your budget is, what kind of tools you have access to, what kind of materials you feel comfortable working with, and what kind of strength, durability, and weight characteristics you need. Titanium is a wonderfully lightweight and strong material, but it’s also expensive and can be difficult to work with. Steel is strong and durable, and much cheaper and easier to weld than titanium, but heavy. Aluiminum is lightweight and easy to work with, but less durable than either steel or titanium. Carbon fiber is very ligthweight and stiff, but expensive and difficult to work with, and not all that durable when it comes to cutting or abrasion. Plastics might also be an option - some of the more modern materials have fairly good strength-to-weight, and are much easier than metal to cut and drill.
What about sponge?
Old fried chicken bones, twine, christmas lights and most of a 1986 Fierro.