I have 25s on my geared bike. I don't race. I do like to go fast. 25mm makes the most sense to me because it's a little more forgiving than the smaller, lighter tires that are available.

The fixie has 32mm tires. Steel frame. It's the SUV. It's the most fun on hardpack, fireroads, and smooth singletrack. I take it out in 2-3" of fresh snow, and it makes me feel guilty it's so much fun.