What is the strongest lithium-ion battery?

Assuming you mean physical strength, the physical strength of a Li-ion cell comes from its packaging. The active material on the electrode (Li-oxide and other partner materials) are in very small particle sizes, and is applied to a thin metal foil in a paste form with a binder. When dried, this material does not have much mechanical capabilities by itself.

In a cylindrical cell, many layers of the metal foils with active material coating are tightly wound into a rigid cylinder. Then a metal casing is over-wrapped with metal end-caps crimped tightly together. All these various metal components contribute to the mechanical strengths of a cell. See illustration below:

The pouch cell or Li-Poly is simply the active materials on metal foils sandwiched together like a booklet sealed inside a polymer film laminated with aluminum foil. See generic illustration below:

The 3rd kind of cell design is called a prismatic cell. The active cell material is rolled up in a flat shape and placed in a metal can as show below. This provides much better mechanical strengths and protection as compared to pouch cells. It is also much easier to assemble into a larger pack and for maintenance.

If you meant which Li-ion chemistry has the highest specific-energy, then it is the NCA at this time; 254 Wh/kg. The LFP is only around 120–140 Wh/kg.

If you meant which Li-ion chemistry has the highest cell voltage, then it is the LCO chemistry; 3.7V/cell. The LFP is only around 3.2V/cell.

If you meant strength to be something else, then please clarify further.