I tend to use a yellow filter (mostly) with Efke 25. It seems to do the best job in general, but a lot depends on the scene. Without a filter, skies will be very pale and washed out, nearly white in the print. A polarizer is also a good choice for skies or glare, but the trade-off is in two stops less exposure, some scenes will not tolerate this slow of a shutter for landscapes. As Vinny has mentioned, an orange filter is about as dark as you will want to try.
Try a yellow #8 and see what you get. It helps with contrast, in the shadows, highlights and cloud separation. Try doing your shots with and without a filter so you can get a feel for the outcome in a print. Do one without, with a yellow, an orange and with a polarizer. This should let you see what works best in a given setting. One roll of 35mm should do it. tim
P.S. Most recent upload in my gallery was with efke 25, pyrocat-hd & a polarizer (4x5 at San Xavier mission in Tucson)