|My Model Set in a Toolbox|
In a Plastic Case ($24)
In a Corrugated Box ($22)
In a Plastic Bag ($21)
These model sets are plastic, durable, and easily constructed. I particularly like how easy it is to rotate around bonds once you get them worked in a little. They superbly demonstrate how a cyclohexane chair flips from one chair conformation to another. That's a tough concept to understand until you can feel it in your hands.
I started with Kit #1A in grad school (I'm old; these fancy plastic models didn't exist when I was in college) and have slowly added pieces over the years so that I could build larger molecules like a protein alpha helix and a full turn of DNA. When I need to understand a complex structure, a physical model is still my first tool.