The question could be considered fairly subjective when asking "best for the money." I would suggest a segmental retaining wall (dry-stacked concrete block) system from a reputable manufacturer that has info readily available for a 6' high wall. I'm sure it will cost a lot more than the wood wall your replacing, but if installed correctly, will last many times longer than the wood. In the last week, there's much discussion on different brands of SRW. Check out some of those threads below for more info or do a search on here.
If you oak lasted 3 years and now you are considering replacement, you have some serious moisture/drainage problems.
Obviously, a SRW (segmental retaining wall) wall is the best way to go in the long run, especially if you have those sort of site problems. A little grading and a perforated drain pipe behind the wall is cheap and will help however you rebuild.
If you are set on using wood (any type), plan to replace it frequently unless you solve the real problem (moisture control). You can now get reasonably cheap RR ties that were torn out because they are usually not sold because of the preference for a more attractive appearance.
A concrete wall with a footing is out of the question because of the construction problems and guarantee of cracks due to Mother Nature.
In any case, a SRW wall will be better in the end.
Your 6' height (normally there is a 4' height limit for no permit) could get a little tricky to build or even get a permit for, especially if you are just stacking up wet fire wood and hoping it is strong enough. Most areas require and engineered design (not that expensive since they only look at the soil and go to tables), but it is protection for your investment in time and materials.
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