Best timber for raised beds?

I’m looking to build some raised beds up against a shed and wondered what is the best type of timber to use? I’m looking for something long-lasting, I don’t want to be rebuilding it every couple of years! It will be mostly used for flowers so I’m not worried about preservatives leaching into soil etc. I have beds made of railway sleepers elsewhere in the garden but this will be a very long narrow bed so can’t use them in this instance.
Thanks in advance for any advice!