Currently the most common hardware is 64 bits architectures PC — Intel / AMD — also know as x86_64.
Choosing another architecture certainly means that you already know what you are doing.
Please read also:
OMLx 6.0 Release Notes
OMLx 6.0 Errata
If in doubt, go with the Default Download at SourceForge:
or:
Select the release in the list, it should automatically open a download page from a mirror nearby your location.
OpenMandriva mirrors download page: the current images with their SHA256 sums, served from a mirror near you. Each image also has a .meta4 (Metalink) link for download managers such as aria2, KGet or wget2.
Click on the desired version to open the details, and download torrent or iso file from the links in the right panel.