Summer. Nothing better than something yummy to grab at every plant.

All four and concurrently.
The best designers create an interesting and viable landscape that holds your attention 365 days a year.

The only exception I would note is if you are designing for a summer house that only is visited in the summer or a ski lodge that only is used in the winter.
But besides that, all the seasons and their transitional times should be taken into account, its a mark of a good designer.

Someone posted in another thread, to visit your nursery every month, and buy 3 of something in full bloom, so you always have color in your yard. Of course, you have to consider size, but I thought that was a good plan.

We just bought a house that was seriously neglected and overgrown. It has good bones, but that's about it! All your posts interest me. I will post photos one of these days to get your suggestions on curb appeal. That's just the beginning. But start with that for sure!

The subtopic of intended season of use for a garden is also very useful to keep in mind when doing planting design. As Michelle stated, it makes no sense to design outside the season of use, unless the client has requested that it also please the neighborhood, not just them as they use it. Living in a zone 9b/10a climate zone, I like designing for fall into spring as the major plant interest myself. It is just as easy to have a garden appear to be in full bloom here in winter as in the spring, and there is also less need to water things in our winter rainy season. No snow and bare deciduous twigs and dormant perennials as the default landscape here in the SF Bay Area, unless you prefer that look, and some people do.

Boy if I lived in zone 9b, I don't think I would even recognize that the seasons even changed. Obviously they do, but for us in the upper Midwest we would hardy recognize winter. So obviously zone consideration is a consideration.

In Chicago in the winter we stay in buildings viewing our landscapes mostly from inside, while in warmer climates people may actually get out into the landscape more in the winter (i.e. Tucson, AZ).

My general philosophy is design for winter 1st and foremost (5 months of dormant season with not leaves), then

Summer when people will be out in the season the most, then

Fall (include plants with great fall color and it almost designs itself,

Spring almost takes care of itself (not really but it is really easy to make spring look great).

Here is a link that might be useful: Plan view of Front yard design for winter interest