To add to franks list:
Thermo ash (Ash that has gone through a process similar to torrefaction you will only be able to get it from a flooring or decking supplier, cumaru, teak (if you can get it) will all last at least 25 years in western climates (if not too extreme like Florida) and around 10 in tropical climes. Black locust is that same class.
With all of these sapwood, the pith, the outermost and innermost heartwood rings and defects all are less durable. Also, as they will get wet and then dry a lot, quarter sawn wood is better than flat sawn for stability. You don’t want rift sawn because it can twist and rip out your fastenings.
Yew is very durable too. I seem to remember you having quite a lot. Might want to try that. (Yew is also in that same class of durability as black locust)