Which LEGO set is best suited to build robots?
Your question is not so clear, but it seems like you want to build robots and you want Technic pieces. Then Lego Mindstorms is the product you should buy. While not formally part of the Technic range, it contains only Technic elements in addition to the special electronic Mindstorms parts.

You can also take a look at Lego BOOST, a lightweight variant of Mindstorms, but that is not as much oriented around Technic pieces.

If you don't want the electronics, then you could consider the Bucket Excavator which contains a large number of elements for a fair price. Alternatively maybe you should also look at Meccano.