We are looking at doing a remodel next spring and putting in a walk out basement. I am looking at a 7 foot retaing wall with lots of room to tier it if it is the best way. Question I have is what's the best method and what material would work the best. I am in Wisconsin so frost is definately a consideration.
Look into segmental retaining walls (SRWs). They are paticilarly weel suited to cold weather. They never need a concrete footing below frost and and be either straight or curved and terraced.
Look at the sites for Allan Blco, Anchor Wall systems, Keystone and Versalok. All are somewhat different, but use the same general design and structural concept. They have good sites and information and the walls are used both domestically and internationally in addition to be standards for many municipalites.
From your location, you may see some different types of "knock-off" systems that look similar, but do not have all the features.
Like all types of retaining walls, anytime you get over 4' high or so, engineering would be required, but usually is pretty economical because of the simplicity and common use.