For many years I used Yellow, Dark Yellow and Orange filters to add contrast and enhance skies. I occasionally use yellow-green and green if there are lots of green trees and shrubs in the shot and not much sky. I hadn’t used red filters until recently (2014). B+W, the manufacturer of most of my filters, has deleted dark yellow, and green filters from their range so I have changed my standard filters to Yellow, Orange and Red. I also use infra-red filters (Heliopan 715, similar to a Hoya R72) with Ilford SFX film.
The dark yellow was my favourite until the B+W range change from 2014 since a lot of shots have trees in and this does not darken them as much as an orange but still darkens blue skies quite a bit more than a yellow.