Shareable partitions in increasing order of issues: swap, boot, tmp, home. Any general 'data' partition for your own files would be fine. Efficient use of space is probably not to make too big a swap or download or install too much junk.
Also, installing multiple distros of course wastes space. But that doesn't stop me having five.
All I share is swap and a giant data section I mount under various /home's, but my arrangement is a mess. Anything else I need I mount under a /mnt/hd or wherever when the occasion arises.