On really hot days, I really like Sol with salt and lime (Chelada). Very refreshing. I'm here in Mexico, and another good (darker) beer is Indio. Very good, IMO.
A much as I love Guinness, I just can't do it on hot days, especially if I have work to do. Just too filling. Of course, Yeungling is great too, but a little too 'heavy' on very hot days. Jeesh, sounds like a wimpy post, but I only weigh 150 lbs, so every 12 ounces counts...