It depends what your working conditions are. I would love to use thick aluminium for my chassis with hardox armour over it but the only way I could achieve that is if someone else built it for me, because I don't have good enough facilities to cut/drill those materials with any degree of alignment or precision. The cost of getting someone to do it would make for a very expensive build too.

So I opt a lot for plastics. They cost less, are easier to work with and are readily available from the likes of Direct Plastics in a range of thicknesses. And a well-built robot made from plastic can be just as good as any built from steel, aluminium or titanium. All you need is a jig-saw (but can also be cut with a hand-saw), a cordless drill and a workbench or unit to work on and you can build a robot quite easily out of the stuff.

Stiffer materials such as Nylon and UHMWPE can work well for chassis construction, while the likes of HDPE is a good source of cheap, thick armour that resists spinning blades quite effectively. You can also use HDPE for the main chassis construction but it is not as strong when it comes to load-bearing parts and can deform more easily.

And if you still wanted some metal on your robot, you can put sheets on top of HDPE that wouldn't need to be as thick since you have the plastic behind it as another layer.