There's so much out there it's hard to choose from. My concerns are cost, drinkability, and minimizing of questionable ingredients.
It's odd that you don't mention what is probably the most important concern you'll have here -- tolerability (unless that's what you mean by drinkability, althoguh personally I differentiate the two.) Perhaps you're making the assumption that anyone will be smart enough to figure that that's assumed, but I assure you, sir, I for one have been told I challenge anyone's assumption of adult intelligence!
Seriously, though -- tolerability is the biggie for me. Drinkability is something different in my mind -- drinkability is how much I actually like it. Tolerability is whether it's going to make my stomach rebel nine hours into a 12 hour race. For example, I actually like the taste of Perpetuem's Caffe Latte mix, but if I drink just that in a long race, my stomach is going to explode about halfway through. I, personally, just can't tolerate the protein. Sustained Energy, which is unflavored, doesn't have that same protein component and I can exist almost exclusively on that for a 24 hour race (as long as I can supplement with water every few laps.)
When it comes to stuff like this, I don't consider cost at all. I just don't care -- if something works, I use it because I spend way too much time in the saddle to let the relative cost be an issue. But I understand that may not be the case for many other people. I just can't help you there.
Of the different mixes I have drank int he past (Cytomax, Perpetuem, Sustained Energy, Zym), I like the taste of the Zym the best. Very "fizzy". I don't like the Hammer Fizz (whcih is supposed to be the same) -- too chalky.
The way I put my "big three" (Sustained Energy, Water and Zym) together depends on the type of ride I'm doing -- it varies by distance and whether its a training ride or a race. Whenever I'm in doubt, though, I err on the side of simpler: water is the best option if I don't know which. After that, it's Sustained Energy and then Zym. I like the Zym the best, but it's effervescent, and after 6 hours on a bike in the heat, effervescence can wreak havoc on your stomach.