I've recently started doing 4 infantry figures on 40mmx40mm bases. That sticks with the popular 20mm square per figure basing, but keeps them in groups. Larger critters go one per 40mm square. Cavalry go 2 per 50x50 square, as do larger creatures that won't fit on a 40x40.
Previously I had been doing 3 figures per 20mmx60mm base, along with some single 20mmx20mm based figures.
I've gave it way too much thought and came to the conclusion that no matter what, there isn't a "best" option, and no matter how well a given basing scheme may work in one situation/rules, it will be awful in others. Pros & cons to each basing scheme depends on how you're going to use them, what rules, etc.