Whatever is cheapest that doesn't taste shite. I tend to use Torq powder since, if I'm riding enough to worry about hydration I probably should be fueling too, but I do have tabs for occasional use - I buy whatever is on special. In my case it was some High5 and some Wiggle tabs.
What are the current forum recommendations for hydration tables (no caffeine), NUUN, High 5, SIS or something else, e.g., Dioralyte sachets?