Best electrolyte drink I've encountered is pedialyte which is available in every pharmacy.
I do realize the discussion is actually about "sports drinks", but...
Pedialyte is nothing more or less than the old World Health Organization's oral rehydration formula sold pre-mixed for suburban convenience at much higher cost than the WHO stuff (which has also been replaced with a reduced osmolarity formulation since Pedialyte hit the market).
It's intended to replace fluids lost to diarrhea, especially from such things as cholera. The sugars and salts ("electrolytes") are in it because they cause much faster absorption and better retention than water drunk alone, not really to replace lost electrolytes. It is not intended to replace fluids lost to sweating, or really even vomiting (though many use it for that). It's very easy to make a highly-effective "close enough" version using nothing more than table salt, sugar (of many different types), and water.
If you have access to a pharmacy or chemical supply (for potassium chloride and citric acid), it's easy to duplicate it exactly. WHO supplies it in packets to make 1 liter for about a dime, if you buy them by the pallet. Abbott is making a nice profit on it in Pedialyte bottles with awful artificial flavors added.
For short-term use, WHO officially recommends starting with homemade ORT solution (6 teaspoons of sugar, 1/2 teaspoon of table salt, and 1 liter of water - for those without US measuring spoon, a teaspoon is now officially defined as 5mL, so by volume that's 30mL of sugar and 2.5mL of salt) at the start of diarrhea before dehydration has begun.
rehydrate.org is a good source of information about this.
Pedialyte isn't for sports use
Pedialyte is convenient
Pedialyte is rarely necessary as long as you have access to sugar, salt, and clean water
Pedialyte might be a good choice for someone who actually has cholera or some other form of severe diarrhea and has good access to Western grocery stores but not, for some reason, access to an actual hospital