Originally Posted by abc123
First, it depends on what fiber/weight/weave and color(s) of odd jackets you like to wear.
That said, in my view, gray flannel should always be #1. This is THE odd trouser, and perfect with everything. If you can afford to go nuts, it's nice to have them in various weights (say, at least two: 16 ounce and 10 ounce) and colors (medium to light look best, I think). You might also consider a gray twill worsted. Not exciting, but it can be useful. After that, brown cavalry twill is nice, as is tan gabardine. There a lot of cotton choices. For cooler weather, moleskin and corduroy are great, and work in many, many colors. I happen to like olive for corduroy, because it looks good in itself, and I really don't like olive wool. For warm weather, light tan or cream brushed khaki and linen are nice.