best toy construction kits ?

Projects General Guidance
1

Hi !
What are, in your opinion, the best construction kits for simple project (robots for example) prototypying ? I'm thinking about lego, meccanos, K'nex, etc... what do you like to use ?

2

The best out there (ok - there's some micro T-slot out there that looks interesting, too), in my opinion:

Unfortunately, it makes Lego look inexpensive.