Of the four, I would choose Micron for the best risk-adjusted exposure, although SanDisk has the most explosive upside.

My ranking: Micron > SK Hynix > SanDisk > Western Digital.

Micron: My preferred balance of HBM/DRAM exposure, AI demand and valuation. Druckenmiller's Q2 exit is worth noting, but I would not treat one fund manager's portfolio decision as a fundamental signal. 

SK Hynix: Probably the strongest pure HBM beneficiary, but you are paying for that leadership. It is less directly exposed to the SanDisk/NAND thesis.

SanDisk: Highest upside, highest risk. The Investor Day genuinely changes the story: eight NBM agreements covering roughly half of FY27 and two-thirds of FY28 capacity provide unusually strong demand visibility. Management is targeting mid-to-high-teens revenue growth and ~80% gross margins for FY28-30. But after a 35% weekly surge, I would not chase it. The yen risk is legitimate because of its Japanese manufacturing exposure. 

Western Digital: More of a secondary play. I would rather own the companies with clearer exposure to the most profitable parts of the memory cycle.

The interesting part about SanDisk is that the thesis is becoming less about simply calling the NAND cycle correctly and more about contracted capacity, pricing power and HBF potentially creating a new AI-storage market. That is a much better business model if management can execute. 

If buying today: Micron.

If willing to accept substantially more volatility: SanDisk, but preferably after a pullback rather than after +35%.

If I already owned SanDisk: I would be more inclined to hold than chase.