It depends on a combination of factors, so I don't look for the same things every time. For example, there are times when I look for loose tables with high percentage seeing flops. Other times, however, I feel like playing less active tables. Even though they're less profitable over the long run, the difference isn't always enough for me to min-max.
Other times, I look for tables with opponents against whom I have a solid edge, especially if I can get position a seat or two to their left.