While mid grey flannels are certainly the most versatile odd trousers, I don't think they're always the best. A brown tweed jacket can sometimes look much better with deep forest green corduroy trousers IMHO. It's F/W casual perfection IMO.
My favorite cotton trousers in falling order (to wear with SC's):
1. Corduroy, the essential partner for a lot of tweed jackets.
2. Moleskin, actually just as versatile as corduroy, but not as big a personal favourite.