Where is the best place for LUKS to sit in a RAID/LVM/LUKS stack?
I would make the RAID array first because when you will change a disk of the array, it will be tranparent for LUKS and LVM.
If there is no need to keep some partition unencrypted (except for the /boot partition), I would setup LUKS in second because I don't want to have to setup LUKS every time I make a change in my partitions.